php-doc-en/language/namespaces.xml

<?xml version="1.0" encoding="utf-8"?>
<!-- $Revision$ -->
<chapter xml:id="language.namespaces" xmlns="http://docbook.org/ns/docbook"
 version="1.1">
 <title>Namespaces</title>

 <sect1 xml:id="language.namespaces.rationale">
  <title>Namespaces overview</title>
  <?phpdoc print-version-for="namespaces"?>
  <simpara>
   What are namespaces? In the broadest definition namespaces are a way of encapsulating
   items.  This can be seen as an abstract concept in many places. For example, in any
   operating system directories serve to group related files, and act as a namespace for
   the files within them. As a concrete example, the file <literal>foo.txt</literal> can
   exist in both directory <literal>/home/greg</literal> and in <literal>/home/other</literal>,
   but two copies of <literal>foo.txt</literal> cannot co-exist in the same directory. In
   addition, to access the <literal>foo.txt</literal> file outside of the
   <literal>/home/greg</literal> directory, we must prepend the directory name to the file
   name using the directory separator to get <literal>/home/greg/foo.txt</literal>. This
   same principle extends to namespaces in the programming world.
  </simpara>

  <simpara>
   In the PHP world, namespaces are designed to solve two problems that authors
   of libraries and applications encounter when creating re-usable code elements
   such as classes or functions:
  </simpara>
  <para>
   <orderedlist>
    <listitem>
     <simpara>
      Name collisions between code you create, and
      internal PHP classes/functions/constants or third-party classes/functions/constants.
     </simpara>
    </listitem>
    <listitem>
     <simpara>
      Ability to alias (or shorten) Extra_Long_Names designed to alleviate the first problem,
      improving readability of source code.
     </simpara>
    </listitem>
   </orderedlist>
  </para>
  <simpara>
   PHP Namespaces provide a way in which to group related classes, interfaces, 
   functions and constants.  Here is an example of namespace syntax in PHP:
  </simpara>
  <example>
   <title>Namespace syntax example</title>
   <programlisting role="php">
   <![CDATA[
<?php
namespace my\name; // see "Defining Namespaces" section

class MyClass {}
function myfunction() {}
const MYCONST = 1;

$a = new MyClass;
$c = new \my\name\MyClass; // see "Global Space" section

$a = strlen('hi'); // see "Using namespaces: fallback to global
                   // function/constant" section

$d = namespace\MYCONST; // see "namespace operator and __NAMESPACE__
                        // constant" section
$d = __NAMESPACE__ . '\MYCONST';
echo constant($d); // see "Namespaces and dynamic language features" section
?>
    ]]>
   </programlisting>
  </example>
   <note>
    <simpara>
     Namespace names are case-insensitive.
    </simpara>
   </note>   
  <note>
   <para>
    The Namespace name <literal>PHP</literal>, and compound names starting
    with this name (like <literal>PHP\Classes</literal>) are reserved for internal language use 
    and should not be used in the userspace code. 
   </para>
  </note>
 </sect1>

 <sect1 xml:id="language.namespaces.definition">
  <title>Defining namespaces</title>
  <?phpdoc print-version-for="namespaces"?>
  <para>
   Although any valid PHP code can be contained within a namespace, only the following
   types of code are affected by namespaces: classes (including abstracts and traits), interfaces, functions and constants.
  </para>
  <para>
   Namespaces are declared using the <literal>namespace</literal>
   keyword.  A file containing a namespace must declare the namespace
   at the top of the file before any other code - with one exception: the
   <xref linkend="control-structures.declare" /> keyword.
   <example>
    <title>Declaring a single namespace</title>
    <programlisting role="php">
     <![CDATA[
<?php
namespace MyProject;

const CONNECT_OK = 1;
class Connection { /* ... */ }
function connect() { /* ... */ }

?>
]]>
    </programlisting>
   </example>
   <note>
    <simpara>
     Fully qualified names (i.e. names starting with a backslash) are not allowed in namespace
     declarations, because such constructs are interpreted as relative namespace expressions.
    </simpara>
   </note>
   The only code construct allowed before a namespace declaration is the
   <literal>declare</literal> statement, for defining encoding of a source file.  In addition,
   no non-PHP code may precede a namespace declaration, including extra whitespace:
   <example>
    <title>Declaring a single namespace</title>
    <programlisting role="php">
     <![CDATA[
<html>
<?php
namespace MyProject; // fatal error - namespace must be the first statement in the script
?>
]]>
    </programlisting>
   </example>
  </para>
  <para>
   In addition, unlike any other PHP construct, the same namespace may be defined
   in multiple files, allowing splitting up of a namespace's contents across the filesystem.
  </para>
 </sect1>
 <sect1 xml:id="language.namespaces.nested">
  <title>Declaring sub-namespaces</title>
  <?phpdoc print-version-for="namespaces"?>
  <para>
   Much like directories and files, PHP namespaces also contain the ability to specify
   a hierarchy of namespace names.  Thus, a namespace name can be defined with
   sub-levels:
   <example>
    <title>Declaring a single namespace with hierarchy</title>
    <programlisting role="php">
     <![CDATA[
<?php
namespace MyProject\Sub\Level;

const CONNECT_OK = 1;
class Connection { /* ... */ }
function connect() { /* ... */  }

?>
]]>
    </programlisting>
   </example>
   The above example creates constant <literal>MyProject\Sub\Level\CONNECT_OK</literal>,
   class <literal>MyProject\Sub\Level\Connection</literal> and function
   <literal>MyProject\Sub\Level\connect</literal>.
  </para>
 </sect1>
 <sect1 xml:id="language.namespaces.definitionmultiple">
  <title>Defining multiple namespaces in the same file</title>
  <?phpdoc print-version-for="namespaces"?>
  <para>
   Multiple namespaces may also be declared in the same file.  There are two allowed
   syntaxes.
  </para>
  <para>
   <example>
    <title>Declaring multiple namespaces, simple combination syntax</title>
    <programlisting role="php">
     <![CDATA[
<?php
namespace MyProject;

const CONNECT_OK = 1;
class Connection { /* ... */ }
function connect() { /* ... */  }

namespace AnotherProject;

const CONNECT_OK = 1;
class Connection { /* ... */ }
function connect() { /* ... */  }
?>
]]>
    </programlisting>
   </example>
  </para>
  <para>
   This syntax is not recommended for combining namespaces into a single file.
   Instead it is recommended to use the alternate bracketed syntax.
  </para>
  <para>
   <example>
    <title>Declaring multiple namespaces, bracketed syntax</title>
    <programlisting role="php">
     <![CDATA[
<?php
namespace MyProject {

const CONNECT_OK = 1;
class Connection { /* ... */ }
function connect() { /* ... */  }
}

namespace AnotherProject {

const CONNECT_OK = 1;
class Connection { /* ... */ }
function connect() { /* ... */  }
}
?>
]]>
    </programlisting>
   </example>
  </para>
  <para>
   It is strongly discouraged as a coding practice to combine multiple namespaces into
   the same file.  The primary use case is to combine multiple PHP scripts into the same
   file.
  </para>
  <para>
   To combine global non-namespaced code with namespaced code, only bracketed syntax is
   supported.  Global code should be
   encased in a namespace statement with no namespace name as in:
   <example>
    <title>Declaring multiple namespaces and unnamespaced code</title>
    <programlisting role="php">
     <![CDATA[
<?php
namespace MyProject {

const CONNECT_OK = 1;
class Connection { /* ... */ }
function connect() { /* ... */  }
}

namespace { // global code
session_start();
$a = MyProject\connect();
echo MyProject\Connection::start();
}
?>
]]>
    </programlisting>
   </example>
  </para>
  <para>
   No PHP code may exist outside of the namespace brackets except for an opening
   declare statement.
   <example>
    <title>Declaring multiple namespaces and unnamespaced code</title>
    <programlisting role="php">
     <![CDATA[
<?php
declare(encoding='UTF-8');
namespace MyProject {

const CONNECT_OK = 1;
class Connection { /* ... */ }
function connect() { /* ... */  }
}

namespace { // global code
session_start();
$a = MyProject\connect();
echo MyProject\Connection::start();
}
?>
]]>
    </programlisting>
   </example>
  </para>
 </sect1>
 <sect1 xml:id="language.namespaces.basics">
  <title>Using namespaces: Basics</title>
  <?phpdoc print-version-for="namespaces"?>
  <para>
   Before discussing the use of namespaces, it is important to understand how PHP
   knows which namespaced element your code is requesting.  A simple analogy can be made
   between PHP namespaces and a filesystem.  There are three ways to access a file in a
   file system:
   <orderedlist>
    <listitem>
     <simpara>
      Relative file name like <literal>foo.txt</literal>.  This resolves to
      <literal>currentdirectory/foo.txt</literal> where currentdirectory is the
      directory currently occupied.  So if the current directory is
      <literal>/home/foo</literal>, the name resolves to <literal>/home/foo/foo.txt</literal>.
     </simpara>
    </listitem>
    <listitem>
     <simpara>
      Relative path name like <literal>subdirectory/foo.txt</literal>.  This resolves
      to <literal>currentdirectory/subdirectory/foo.txt</literal>.
     </simpara>
    </listitem>
    <listitem>
     <simpara>
      Absolute path name like <literal>/main/foo.txt</literal>.  This resolves
      to <literal>/main/foo.txt</literal>.
     </simpara>
    </listitem>
   </orderedlist>
   The same principle can be applied to namespaced elements in PHP.  For
   example, a class name can be referred to in three ways:
   <orderedlist>
    <listitem>
     <simpara>
      Unqualified name, or an unprefixed class name like
      <literal>$a = new foo();</literal> or
      <literal>foo::staticmethod();</literal>.  If the current namespace is
      <literal>currentnamespace</literal>, this resolves to
      <literal>currentnamespace\foo</literal>.  If
      the code is global, non-namespaced code, this resolves to <literal>foo</literal>.
     </simpara>
     <simpara>
      One caveat: unqualified names for functions and constants will
      resolve to global functions and constants if the namespaced function or constant
      is not defined.  See <link linkend="language.namespaces.fallback">Using namespaces:
      fallback to global function/constant</link> for details.
     </simpara>
    </listitem>
    <listitem>
     <simpara>
      Qualified name, or a prefixed class name like
      <literal>$a = new subnamespace\foo();</literal> or
      <literal>subnamespace\foo::staticmethod();</literal>.  If the current namespace is
      <literal>currentnamespace</literal>, this resolves to
      <literal>currentnamespace\subnamespace\foo</literal>.  If
      the code is global, non-namespaced code, this resolves to <literal>subnamespace\foo</literal>.
     </simpara>
    </listitem>
    <listitem>
     <simpara>
      Fully qualified name, or a prefixed name with global prefix operator like
      <literal>$a = new \currentnamespace\foo();</literal> or
      <literal>\currentnamespace\foo::staticmethod();</literal>.  This always resolves
      to the literal name specified in the code, <literal>currentnamespace\foo</literal>.
     </simpara>
    </listitem>
   </orderedlist>
  </para>
  <para>
   Here is an example of the three kinds of syntax in actual code:
   <informalexample>
    <simpara>file1.php</simpara>
    <programlisting role="php">
     <![CDATA[
<?php
namespace Foo\Bar\subnamespace;

const FOO = 1;
function foo() {}
class foo
{
    static function staticmethod() {}
}
?>
     ]]>
    </programlisting>
    <simpara>file2.php</simpara>
    <programlisting role="php">
     <![CDATA[
<?php
namespace Foo\Bar;
include 'file1.php';

const FOO = 2;
function foo() {}
class foo
{
    static function staticmethod() {}
}

/* Unqualified name */
foo(); // resolves to function Foo\Bar\foo
foo::staticmethod(); // resolves to class Foo\Bar\foo, method staticmethod
echo FOO; // resolves to constant Foo\Bar\FOO

/* Qualified name */
subnamespace\foo(); // resolves to function Foo\Bar\subnamespace\foo
subnamespace\foo::staticmethod(); // resolves to class Foo\Bar\subnamespace\foo,
                                  // method staticmethod
echo subnamespace\FOO; // resolves to constant Foo\Bar\subnamespace\FOO
                                  
/* Fully qualified name */
\Foo\Bar\foo(); // resolves to function Foo\Bar\foo
\Foo\Bar\foo::staticmethod(); // resolves to class Foo\Bar\foo, method staticmethod
echo \Foo\Bar\FOO; // resolves to constant Foo\Bar\FOO
?>
     ]]>
    </programlisting>
   </informalexample>
  </para>
  <para>
   Note that to access any global
   class, function or constant, a fully qualified name can be used, such as
   <function>\strlen</function> or <classname>\Exception</classname> or
   <literal>\INI_ALL</literal>.
   <example>
    <title>Accessing global classes, functions and constants from within a namespace</title>
    <programlisting role="php">
     <![CDATA[
<?php
namespace Foo;

function strlen() {}
const INI_ALL = 3;
class Exception {}

$a = \strlen('hi'); // calls global function strlen
$b = \INI_ALL; // accesses global constant INI_ALL
$c = new \Exception('error'); // instantiates global class Exception
?>
     ]]>
    </programlisting>
   </example>
  </para>
 </sect1>
 <sect1 xml:id="language.namespaces.dynamic">
  <title>Namespaces and dynamic language features</title>
  <?phpdoc print-version-for="namespaces"?>
  <para>
   PHP's implementation of namespaces is influenced by its dynamic nature as a programming
   language.  Thus, to convert code like the following example into namespaced code:
   <example>
    <title>Dynamically accessing elements</title>
    <simpara>example1.php:</simpara>
    <programlisting role="php">
     <![CDATA[
<?php
class classname
{
    function __construct()
    {
        echo __METHOD__,"\n";
    }
}
function funcname()
{
    echo __FUNCTION__,"\n";
}
const constname = "global";

$a = 'classname';
$obj = new $a; // prints classname::__construct
$b = 'funcname';
$b(); // prints funcname
echo constant('constname'), "\n"; // prints global
?>
    ]]>
    </programlisting>
   </example>
   One must use the fully qualified name (class name with namespace prefix).
   Note that because there is no difference between a qualified and a fully qualified Name
   inside a dynamic class name, function name, or constant name, the leading backslash is
   not necessary.
   <example>
    <title>Dynamically accessing namespaced elements</title>
    <programlisting role="php">
     <![CDATA[
<?php
namespace namespacename;
class classname
{
    function __construct()
    {
        echo __METHOD__,"\n";
    }
}
function funcname()
{
    echo __FUNCTION__,"\n";
}
const constname = "namespaced";

/* note that if using double quotes, "\\namespacename\\classname" must be used */
$a = '\namespacename\classname';
$obj = new $a; // prints namespacename\classname::__construct
$a = 'namespacename\classname';
$obj = new $a; // also prints namespacename\classname::__construct
$b = 'namespacename\funcname';
$b(); // prints namespacename\funcname
$b = '\namespacename\funcname';
$b(); // also prints namespacename\funcname
echo constant('\namespacename\constname'), "\n"; // prints namespaced
echo constant('namespacename\constname'), "\n"; // also prints namespaced
?>
    ]]>
    </programlisting>
   </example>
  </para>
  <para>
   Be sure to read the <link linkend="language.namespaces.faq.quote">note about
   escaping namespace names in strings</link>.
  </para>
 </sect1>
 <sect1 xml:id="language.namespaces.nsconstants">
  <title>namespace keyword and __NAMESPACE__ constant</title>
  <?phpdoc print-version-for="namespaces"?>
  <para>
   PHP supports two ways of abstractly accessing elements within the current namespace,
   the <constant>__NAMESPACE__</constant> magic constant, and the <literal>namespace</literal>
   keyword.
  </para>
  <para>
   The value of <constant>__NAMESPACE__</constant> is a string that contains the current
   namespace name.  In global, un-namespaced code, it contains an empty string.
   <example>
    <title>__NAMESPACE__ example, namespaced code</title>
    <programlisting role="php">
     <![CDATA[
<?php
namespace MyProject;

echo '"', __NAMESPACE__, '"'; // outputs "MyProject"
?>
]]>
    </programlisting>
   </example>
   <example>
    <title>__NAMESPACE__ example, global code</title>
    <programlisting role="php">
     <![CDATA[
<?php

echo '"', __NAMESPACE__, '"'; // outputs ""
?>
]]>
    </programlisting>
   </example>
   The <constant>__NAMESPACE__</constant> constant is useful for dynamically constructing
   names, for instance:
   <example>
    <title>using __NAMESPACE__ for dynamic name construction</title>
    <programlisting role="php">
     <![CDATA[
<?php
namespace MyProject;

function get($classname)
{
    $a = __NAMESPACE__ . '\\' . $classname;
    return new $a;
}
?>
]]>
    </programlisting>
   </example>
  </para>
  <para>
   The <literal>namespace</literal> keyword can be used to explicitly request
   an element from the current namespace or a sub-namespace.  It is the namespace
   equivalent of the <literal>self</literal> operator for classes.
   <example>
    <title>the namespace operator, inside a namespace</title>
    <programlisting role="php">
     <![CDATA[
<?php
namespace MyProject;

use blah\blah as mine; // see "Using namespaces: Aliasing/Importing"

blah\mine(); // calls function MyProject\blah\mine()
namespace\blah\mine(); // calls function MyProject\blah\mine()

namespace\func(); // calls function MyProject\func()
namespace\sub\func(); // calls function MyProject\sub\func()
namespace\cname::method(); // calls static method "method" of class MyProject\cname
$a = new namespace\sub\cname(); // instantiates object of class MyProject\sub\cname
$b = namespace\CONSTANT; // assigns value of constant MyProject\CONSTANT to $b
?>
]]>
    </programlisting>
   </example>
   <example>
    <title>the namespace operator, in global code</title>
    <programlisting role="php">
     <![CDATA[
<?php

namespace\func(); // calls function func()
namespace\sub\func(); // calls function sub\func()
namespace\cname::method(); // calls static method "method" of class cname
$a = new namespace\sub\cname(); // instantiates object of class sub\cname
$b = namespace\CONSTANT; // assigns value of constant CONSTANT to $b
?>
]]>
    </programlisting>
   </example>
  </para>
 </sect1>

 <sect1 xml:id="language.namespaces.importing">
  <title>Using namespaces: Aliasing/Importing</title>
  <?phpdoc print-version-for="namespaces"?>
  <para>
   The ability to refer to an external fully qualified name with an alias, or importing,
   is an important feature of namespaces.  This is similar to the
   ability of unix-based filesystems to create symbolic links to a file or to a directory.
  </para>
  <para>
   PHP can alias(/import) constants, functions, classes, interfaces, and namespaces.
  </para>
  <para>
   Aliasing is accomplished with the <literal>use</literal> operator.
   Here is an example showing all 5 kinds of importing:
   <example>
    <title>importing/aliasing with the use operator</title>
    <programlisting role="php">
     <![CDATA[
<?php
namespace foo;
use My\Full\Classname as Another;

// this is the same as use My\Full\NSname as NSname
use My\Full\NSname;

// importing a global class
use ArrayObject;

// importing a function
use function My\Full\functionName;

// aliasing a function
use function My\Full\functionName as func;

// importing a constant
use const My\Full\CONSTANT;

$obj = new namespace\Another; // instantiates object of class foo\Another
$obj = new Another; // instantiates object of class My\Full\Classname
NSname\subns\func(); // calls function My\Full\NSname\subns\func
$a = new ArrayObject(array(1)); // instantiates object of class ArrayObject
// without the "use ArrayObject" we would instantiate an object of class foo\ArrayObject
func(); // calls function My\Full\functionName
echo CONSTANT; // echoes the value of My\Full\CONSTANT
?>
]]>
    </programlisting>
   </example>
   Note that for namespaced names (fully qualified namespace names containing
   namespace separator, such as <literal>Foo\Bar</literal> as opposed to global names that
   do not, such as <literal>FooBar</literal>), the leading backslash is unnecessary and not
   recommended, as import names
   must be fully qualified, and are not processed relative to the current namespace.
  </para>
  <para>
   PHP additionally supports a convenience shortcut to place multiple use statements
   on the same line
   <example>
    <title>importing/aliasing with the use operator, multiple use statements combined</title>
    <programlisting role="php">
     <![CDATA[
<?php
use My\Full\Classname as Another, My\Full\NSname;

$obj = new Another; // instantiates object of class My\Full\Classname
NSname\subns\func(); // calls function My\Full\NSname\subns\func
?>
]]>
    </programlisting>
   </example>
  </para>
  <para>
   Importing is performed at compile-time, and so does not affect dynamic class, function
   or constant names.
   <example>
    <title>Importing and dynamic names</title>
    <programlisting role="php">
     <![CDATA[
<?php
use My\Full\Classname as Another, My\Full\NSname;

$obj = new Another; // instantiates object of class My\Full\Classname
$a = 'Another';
$obj = new $a;      // instantiates object of class Another
?>
]]>
    </programlisting>
   </example>
  </para>
  <para>
   In addition, importing only affects unqualified and qualified names.  Fully qualified
   names are absolute, and unaffected by imports.
   <example>
    <title>Importing and fully qualified names</title>
    <programlisting role="php">
     <![CDATA[
<?php
use My\Full\Classname as Another, My\Full\NSname;

$obj = new Another; // instantiates object of class My\Full\Classname
$obj = new \Another; // instantiates object of class Another
$obj = new Another\thing; // instantiates object of class My\Full\Classname\thing
$obj = new \Another\thing; // instantiates object of class Another\thing
?>
]]>
    </programlisting>
   </example>
  </para>
  <sect2 xml:id="language.namespaces.importing.scope">
   <title>Scoping rules for importing</title>
   <para>
    The <literal>use</literal> keyword must be declared in the 
    outermost scope of a file (the global scope) or inside namespace 
    declarations. This is because the importing is done at compile 
    time and not runtime, so it cannot be block scoped. The following 
    example will show an illegal use of the <literal>use</literal> 
    keyword:
   </para>
   <para>
    <example>
     <title>Illegal importing rule</title>
     <programlisting role="php">
<![CDATA[
<?php
namespace Languages;

function toGreenlandic()
{
    use Languages\Danish;

    // ...
}
?>
]]>
     </programlisting>
    </example>
   </para>
   <note>
    <para>
     Importing rules are per file basis, meaning included files will 
     <emphasis>NOT</emphasis> inherit the parent file's importing rules.
    </para>
   </note>
  </sect2>
  <sect2 xml:id="language.namespaces.importing.group">
   <title>Group <literal>use</literal> declarations</title>
   <para>
    Classes, functions and constants being imported from
    the same &namespace; can be grouped together in a single &use.namespace;
    statement.
   </para>
   <informalexample>
    <programlisting role="php">
<![CDATA[
<?php

use some\namespace\ClassA;
use some\namespace\ClassB;
use some\namespace\ClassC as C;

use function some\namespace\fn_a;
use function some\namespace\fn_b;
use function some\namespace\fn_c;

use const some\namespace\ConstA;
use const some\namespace\ConstB;
use const some\namespace\ConstC;

// is equivalent to the following groupped use declaration
use some\namespace\{ClassA, ClassB, ClassC as C};
use function some\namespace\{fn_a, fn_b, fn_c};
use const some\namespace\{ConstA, ConstB, ConstC};
]]>
    </programlisting>
   </informalexample>
  </sect2>
 </sect1>
 <sect1 xml:id="language.namespaces.global">
  <title>Global space</title>
  <?phpdoc print-version-for="namespaces"?>
  <para>
   Without any namespace definition, all class and function definitions are
   placed into the global space - as it was in PHP before namespaces were
   supported. Prefixing a name with <literal>\</literal> will specify that 
   the name is required from the global space even in the context of the 
   namespace.
   <example>
    <title>Using global space specification</title>
    <programlisting role="php">
     <![CDATA[
<?php
namespace A\B\C;

/* This function is A\B\C\fopen */
function fopen() { 
     /* ... */
     $f = \fopen(...); // call global fopen
     return $f;
} 
?>
    ]]>
    </programlisting>
   </example>
  </para>
 </sect1>
 <sect1 xml:id="language.namespaces.fallback">
  <title>Using namespaces: fallback to global function/constant</title>
  <?phpdoc print-version-for="namespaces"?>
  <para>
   Inside a namespace, when PHP encounters an unqualified Name in a class name, function or
   constant context, it resolves these with different priorities.  Class names always
   resolve to the current namespace name.  Thus to access internal or non-namespaced
   user classes, one must refer to them with their fully qualified Name as in:
   <example>
    <title>Accessing global classes inside a namespace</title>
    <programlisting role="php">
     <![CDATA[
<?php
namespace A\B\C;
class Exception extends \Exception {}

$a = new Exception('hi'); // $a is an object of class A\B\C\Exception
$b = new \Exception('hi'); // $b is an object of class Exception

$c = new ArrayObject; // fatal error, class A\B\C\ArrayObject not found
?>
    ]]>
    </programlisting>
   </example>
  </para>
  <para>
   For functions and constants, PHP will fall back to global functions or constants
   if a namespaced function or constant does not exist.
   <example>
    <title>global functions/constants fallback inside a namespace</title>
    <programlisting role="php">
     <![CDATA[
<?php
namespace A\B\C;

const E_ERROR = 45;
function strlen($str)
{
    return \strlen($str) - 1;
}

echo E_ERROR, "\n"; // prints "45"
echo INI_ALL, "\n"; // prints "7" - falls back to global INI_ALL

echo strlen('hi'), "\n"; // prints "1"
if (is_array('hi')) { // prints "is not array"
    echo "is array\n";
} else {
    echo "is not array\n";
}
?>
    ]]>
    </programlisting>
   </example>
  </para>
 </sect1>

 <sect1 xml:id="language.namespaces.rules">
  <title>Name resolution rules</title>
  <?phpdoc print-version-for="namespaces"?>
  <para>
   For the purposes of these resolution rules, here are some important definitions:
   <variablelist>
    <title>Namespace name definitions</title>
    <varlistentry>
     <term>Unqualified name</term>
     <listitem>
      <para>
       This is an identifier without a namespace separator, such as <literal>Foo</literal>
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term>Qualified name</term>
     <listitem>
      <para>
       This is an identifier with a namespace separator, such as <literal>Foo\Bar</literal>
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term>Fully qualified name</term>
     <listitem>
      <para>
       This is an identifier with a namespace separator that begins with a
       namespace separator, such as <literal>\Foo\Bar</literal>. The namespace
       <literal>\Foo</literal> is also a fully qualified name.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term>Relative name</term>
     <listitem>
      <para>
       This is an identifier starting with <literal>namespace</literal>, such as
       <literal>namespace\Foo\Bar</literal>.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </para>
  <para>
   Names are resolved following these resolution rules:
   <orderedlist>
    <listitem>
     <simpara>
      Fully qualified names always resolve to the name without leading namespace separator.
      For instance <literal>\A\B</literal> resolves to <literal>A\B</literal>.
     </simpara>
    </listitem>
    <listitem>
     <simpara>
      Relative names always resolve to the name with <literal>namespace</literal> replaced by
      the current namespace. If the name occurs in the global namespace, the
      <literal>namespace\</literal> prefix is stripped. For example <literal>namespace\A</literal>
      inside namespace <literal>X\Y</literal> resolves to <literal>X\Y\A</literal>. The same name
      inside the global namespace resolves to <literal>A</literal>.
     </simpara>
    </listitem>
    <listitem>
     <simpara>
      For qualified names the first segment of the name is translated according to the current
      class/namespace import table. For example, if the namespace <literal>A\B\C</literal> is
      imported as <literal>C</literal>, the name <literal>C\D\E</literal> is translated to
      <literal>A\B\C\D\E</literal>.
     </simpara>
    </listitem>
    <listitem>
     <simpara>
      For qualified names, if no import rule applies, the current namespace is prepended to the
      name. For example, the name <literal>C\D\E</literal> inside namespace <literal>A\B</literal>,
      resolves to <literal>A\B\C\D\E</literal>.
     </simpara>
    </listitem>
    <listitem>
     <simpara>
      For unqualified names, the name is translated according to the current import table for the
      respective symbol type. This means that class-like names are translated according to the
      class/namespace import table, function names according to the function import table and
      constants according to the constant import table. For example, after
      <literal>use A\B\C;</literal> a usage such as <literal>new C()</literal> resolves to the name
      <literal>A\B\C()</literal>. Similarly, after <literal>use function A\B\fn;</literal> a usage
      such as <literal>fn()</literal> resolves to the name <literal>A\B\fn</literal>.
     </simpara>
    </listitem>
    <listitem>
     <simpara>
      For unqualified names, if no import rule applies and the name refers to a class-like symbol,
      the current namespace is prepended. For example <literal>new C()</literal> inside namespace
      <literal>A\B</literal> resolves to name <literal>A\B\C</literal>.
     </simpara>
    </listitem>
    <listitem>
     <simpara>
      For unqualified names, if no import rule applies and the name refers to a function or constant
      and the code is outside the global namespace, the name is resolved at runtime.
      Assuming the code is in namespace <literal>A\B</literal>, here is how a call to function
      <literal>foo()</literal> is resolved:
     </simpara>
      <orderedlist>
       <listitem>
        <simpara>
         It looks for a function from the current namespace:
         <literal>A\B\foo()</literal>.
        </simpara>
       </listitem>
       <listitem>
        <simpara>
         It tries to find and call the <emphasis>global</emphasis> function
         <literal>foo()</literal>.
        </simpara>
       </listitem>
      </orderedlist>
    </listitem>
   </orderedlist>
  </para>
  <example>
   <title>Name resolutions illustrated</title>
   <programlisting role="php">
<![CDATA[
<?php
namespace A;
use B\D, C\E as F;

// function calls

foo();      // first tries to call "foo" defined in namespace "A"
            // then calls global function "foo"

\foo();     // calls function "foo" defined in global scope

my\foo();   // calls function "foo" defined in namespace "A\my"

F();        // first tries to call "F" defined in namespace "A"
            // then calls global function "F"

// class references

new B();    // creates object of class "B" defined in namespace "A"
            // if not found, it tries to autoload class "A\B"

new D();    // using import rules, creates object of class "D" defined in namespace "B"
            // if not found, it tries to autoload class "B\D"

new F();    // using import rules, creates object of class "E" defined in namespace "C"
            // if not found, it tries to autoload class "C\E"

new \B();   // creates object of class "B" defined in global scope
            // if not found, it tries to autoload class "B"

new \D();   // creates object of class "D" defined in global scope
            // if not found, it tries to autoload class "D"

new \F();   // creates object of class "F" defined in global scope
            // if not found, it tries to autoload class "F"

// static methods/namespace functions from another namespace

B\foo();    // calls function "foo" from namespace "A\B"

B::foo();   // calls method "foo" of class "B" defined in namespace "A"
            // if class "A\B" not found, it tries to autoload class "A\B"

D::foo();   // using import rules, calls method "foo" of class "D" defined in namespace "B"
            // if class "B\D" not found, it tries to autoload class "B\D"

\B\foo();   // calls function "foo" from namespace "B"

\B::foo();  // calls method "foo" of class "B" from global scope
            // if class "B" not found, it tries to autoload class "B"

// static methods/namespace functions of current namespace

A\B::foo();   // calls method "foo" of class "B" from namespace "A\A"
              // if class "A\A\B" not found, it tries to autoload class "A\A\B"

\A\B::foo();  // calls method "foo" of class "B" from namespace "A"
              // if class "A\B" not found, it tries to autoload class "A\B"
?>
]]>
   </programlisting>
  </example>
 </sect1>
 <sect1 xml:id="language.namespaces.faq">
  <title>FAQ: things you need to know about namespaces</title>
  <?phpdoc print-version-for="namespaces"?>
  <para>
   This FAQ is split into two sections: common questions, and some specifics of
   implementation that are helpful to understand fully.
  </para>
  <para>
   First, the common questions.
   <orderedlist>
    <listitem>
     <simpara>
      <link linkend="language.namespaces.faq.shouldicare">If I don't use namespaces, should
      I care about any of this?</link>
     </simpara>
    </listitem>
    <listitem>
     <simpara>
      <link linkend="language.namespaces.faq.globalclass">How do I use internal or global
      classes in a namespace?</link>
     </simpara>
    </listitem>
    <listitem>
     <simpara>
      <link linkend="language.namespaces.faq.innamespace">How do I use namespaces classes
      functions, or constants in their own namespace?</link>
     </simpara>
    </listitem>
    <listitem>
     <simpara>
      <link linkend="language.namespaces.faq.full">
       How does a name like <literal>\my\name</literal> or <literal>\name</literal>
       resolve?
      </link>
     </simpara>
    </listitem>
    <listitem>
     <simpara>
      <link linkend="language.namespaces.faq.qualified">How does a name like
      <literal>my\name</literal> resolve?</link>
     </simpara>
    </listitem>
    <listitem>
     <simpara>
      <link linkend="language.namespaces.faq.shortname1">How does an unqualified class name
      like <literal>name</literal> resolve?</link>
     </simpara>
    </listitem>
    <listitem>
     <simpara>
      <link linkend="language.namespaces.faq.shortname2">How does an unqualified function
      name or unqualified constant name
      like <literal>name</literal> resolve?</link>
     </simpara>
    </listitem>
   </orderedlist>
  </para>
  <para>
   There are a few implementation details of the namespace implementations
   that are helpful to understand.
   <orderedlist>
    <listitem>
     <simpara>
      <link linkend="language.namespaces.faq.conflict">Import names must not conflict with
      classes defined in the same file.</link>
     </simpara>
    </listitem>
    <listitem>
     <simpara>
      <link linkend="language.namespaces.faq.nested">Nested namespaces are not allowed.
      </link>
     </simpara>
    </listitem>
    <listitem>
     <simpara>
      <link linkend="language.namespaces.faq.quote">Dynamic namespace names (quoted
      identifiers) should escape backslash.</link>
     </simpara>
    </listitem>
    <listitem>
     <simpara>
      <link linkend="language.namespaces.faq.constants">Undefined Constants referenced
      using any backslash die with fatal error</link>
     </simpara>
    </listitem>
    <listitem>
     <simpara>
      <link linkend="language.namespaces.faq.builtinconst">Cannot override special
      constants NULL, TRUE, FALSE, ZEND_THREAD_SAFE or ZEND_DEBUG_BUILD</link>
     </simpara>
    </listitem>
   </orderedlist>
  </para>
  <sect2 xml:id="language.namespaces.faq.shouldicare">
   <title>If I don't use namespaces, should I care about any of this?</title>
   <para>
    No.  Namespaces do not affect any existing code in any way, or any
    as-yet-to-be-written code that does not contain namespaces.  You can
    write this code if you wish:
   </para>
   <para>
    <example>
     <title>Accessing global classes outside a namespace</title>
     <programlisting role="php">
<![CDATA[
<?php
$a = new \stdClass;
?>
]]>
     </programlisting>
    </example>
   </para>
   <para>
    This is functionally equivalent to:
   </para>
   <para>
    <example>
     <title>Accessing global classes outside a namespace</title>
     <programlisting role="php">
<![CDATA[
<?php
$a = new stdClass;
?>
]]>
     </programlisting>
    </example>
   </para>
  </sect2>
  <sect2 xml:id="language.namespaces.faq.globalclass">
   <title>How do I use internal or global classes in a namespace?</title>
   <para>
    <example>
     <title>Accessing internal classes in namespaces</title>
     <programlisting role="php">
<![CDATA[
<?php
namespace foo;
$a = new \stdClass;

function test(\ArrayObject $typehintexample = null) {}

$a = \DirectoryIterator::CURRENT_AS_FILEINFO;

// extending an internal or global class
class MyException extends \Exception {}
?>
]]>
     </programlisting>
    </example>
   </para>
  </sect2>
  <sect2 xml:id="language.namespaces.faq.innamespace">
   <title>
    How do I use namespaces classes, functions, or constants in their own
    namespace?
   </title>
   <para>
    <example>
     <title>Accessing internal classes, functions or constants in namespaces</title>
     <programlisting role="php">
<![CDATA[
<?php
namespace foo;

class MyClass {}

// using a class from the current namespace as a type hint
function test(MyClass $typehintexample = null) {}
// another way to use a class from the current namespace as a type hint
function test(\foo\MyClass $typehintexample = null) {}

// extending a class from the current namespace
class Extended extends MyClass {}

// accessing a global function
$a = \globalfunc();

// accessing a global constant
$b = \INI_ALL;
?>
]]>
     </programlisting>
    </example>
   </para>
  </sect2>
  <sect2 xml:id="language.namespaces.faq.full">
   <title>
     How does a name like <literal>\my\name</literal> or <literal>\name</literal>
     resolve?
   </title>
   <para>
    Names that begin with a <literal>\</literal> always resolve to what they
    look like, so <literal>\my\name</literal> is in fact <literal>my\name</literal>,
    and <literal>\Exception</literal> is <literal>Exception</literal>.
    <example>
     <title>Fully Qualified names</title>
     <programlisting role="php">
<![CDATA[
<?php
namespace foo;
$a = new \my\name(); // instantiates "my\name" class
echo \strlen('hi'); // calls function "strlen"
$a = \INI_ALL; // $a is set to the value of constant "INI_ALL"
?>
]]>
     </programlisting>
    </example>
   </para>
  </sect2>
  <sect2 xml:id="language.namespaces.faq.qualified">
   <title>How does a name like <literal>my\name</literal> resolve?</title>
   <para>
    Names that contain a backslash but do not begin with a backslash like
    <literal>my\name</literal> can be resolved in 2 different ways.
   </para>
   <para>
    If there is
    an import statement that aliases another name to <literal>my</literal>, then
    the import alias is applied to the <literal>my</literal> in <literal>my\name</literal>.
   </para>
   <para>
    Otherwise, the current namespace name is prepended to <literal>my\name</literal>.
   </para>
   <para>
    <example>
     <title>Qualified names</title>
     <programlisting role="php">
<![CDATA[
<?php
namespace foo;
use blah\blah as foo;

$a = new my\name(); // instantiates "foo\my\name" class
foo\bar::name(); // calls static method "name" in class "blah\blah\bar"
my\bar(); // calls function "foo\my\bar"
$a = my\BAR; // sets $a to the value of constant "foo\my\BAR"
?>
]]>
     </programlisting>
    </example>
   </para>
  </sect2>
  <sect2 xml:id="language.namespaces.faq.shortname1">
   <title>How does an unqualified class name like <literal>name</literal> resolve?</title>
   <para>
    Class names that do not contain a backslash like
    <literal>name</literal> can be resolved in 2 different ways.
   </para>
   <para>
    If there is
    an import statement that aliases another name to <literal>name</literal>, then
    the import alias is applied.
   </para>
   <para>
    Otherwise, the current namespace name is prepended to <literal>name</literal>.
   </para>
   <para>
    <example>
     <title>Unqualified class names</title>
     <programlisting role="php">
<![CDATA[
<?php
namespace foo;
use blah\blah as foo;

$a = new name(); // instantiates "foo\name" class
foo::name(); // calls static method "name" in class "blah\blah"
?>
]]>
     </programlisting>
    </example>
   </para>
  </sect2>
  <sect2 xml:id="language.namespaces.faq.shortname2">
   <title>
    How does an unqualified function name or unqualified constant name
    like <literal>name</literal> resolve?
   </title>
   <para>
    Function or constant names that do not contain a backslash like
    <literal>name</literal> can be resolved in 2 different ways.
   </para>
   <para>
    First, the current namespace name is prepended to <literal>name</literal>.
   </para>
   <para>
    Finally, if the constant or function <literal>name</literal> does not exist
    in the current namespace, a global constant or function <literal>name</literal>
    is used if it exists.
   </para>
   <para>
    <example>
     <title>Unqualified function or constant names</title>
     <programlisting role="php">
<![CDATA[
<?php
namespace foo;
use blah\blah as foo;

const FOO = 1;

function my() {}
function foo() {}
function sort(&$a)
{
    \sort($a); // calls the global function "sort"
    $a = array_flip($a);
    return $a;
}

my(); // calls "foo\my"
$a = strlen('hi'); // calls global function "strlen" because "foo\strlen" does not exist
$arr = array(1,3,2);
$b = sort($arr); // calls function "foo\sort"
$c = foo(); // calls function "foo\foo" - import is not applied

$a = FOO; // sets $a to value of constant "foo\FOO" - import is not applied
$b = INI_ALL; // sets $b to value of global constant "INI_ALL"
?>
]]>
     </programlisting>
    </example>
   </para>
  </sect2>
  <sect2 xml:id="language.namespaces.faq.conflict">
   <title>Import names must not conflict with classes defined in the same file.</title>
   <para>
    The following script combinations are legal:
    <informalexample>
     <simpara>file1.php</simpara>
     <programlisting role="php">
     <![CDATA[
<?php
namespace my\stuff;
class MyClass {}
?>
     ]]>
     </programlisting>
     <simpara>another.php</simpara>
     <programlisting role="php">
     <![CDATA[
<?php
namespace another;
class thing {}
?>
     ]]>
     </programlisting>
     <simpara>file2.php</simpara>
     <programlisting role="php">
     <![CDATA[
<?php
namespace my\stuff;
include 'file1.php';
include 'another.php';

use another\thing as MyClass;
$a = new MyClass; // instantiates class "thing" from namespace another
?>
     ]]>
     </programlisting>
    </informalexample>
   </para>
   <para>
    There is no name conflict, even though the class <literal>MyClass</literal> exists
    within the <literal>my\stuff</literal> namespace, because the MyClass definition is
    in a separate file.  However, the next example causes a fatal error on name conflict
    because MyClass is defined in the same file as the use statement.
    <informalexample>
     <programlisting role="php">
     <![CDATA[
<?php
namespace my\stuff;
use another\thing as MyClass;
class MyClass {} // fatal error: MyClass conflicts with import statement
$a = new MyClass;
?>
     ]]>
     </programlisting>
    </informalexample>
   </para>
  </sect2>
  <sect2 xml:id="language.namespaces.faq.nested">
   <title>Nested namespaces are not allowed.</title>
   <para>
    PHP does not allow nesting namespaces
    <informalexample>
     <programlisting role="php">
     <![CDATA[
<?php
namespace my\stuff {
    namespace nested {
        class foo {}
    }
}
?>
     ]]>
     </programlisting>
    </informalexample>
    However, it is easy to simulate nested namespaces like so:
    <informalexample>
     <programlisting role="php">
     <![CDATA[
<?php
namespace my\stuff\nested {
    class foo {}
}
?>
     ]]>
     </programlisting>
    </informalexample>
   </para>
  </sect2>

  <sect2 xml:id="language.namespaces.faq.quote">
   <title>Dynamic namespace names (quoted identifiers) should escape backslash</title>
   <para>
    It is very important to realize that because the backslash is used as an escape character
    within strings, it should always be doubled when used inside a string.  Otherwise
    there is a risk of unintended consequences:
    <example>
     <title>Dangers of using namespaced names inside a double-quoted string</title>
     <programlisting role="php">
      <![CDATA[
<?php
$a = "dangerous\name"; // \n is a newline inside double quoted strings!
$obj = new $a;

$a = 'not\at\all\dangerous'; // no problems here.
$obj = new $a;
?>
      ]]>
     </programlisting>
    </example>
    Inside a single-quoted string, the backslash escape sequence is much safer to use, but it
    is still recommended practice to escape backslashes in all strings as a best practice.
   </para>
  </sect2>
  <sect2 xml:id="language.namespaces.faq.constants">
   <title>Undefined Constants referenced using any backslash die with fatal error</title>
   <para>
    Any undefined constant that is unqualified like <literal>FOO</literal> will
    produce a notice explaining that PHP assumed <literal>FOO</literal> was the value
    of the constant.  Any constant, qualified or fully qualified, that contains a
    backslash will produce a fatal error if not found.
    <example>
     <title>Undefined constants</title>
     <programlisting role="php">
      <![CDATA[
<?php
namespace bar;
$a = FOO; // produces notice - undefined constants "FOO" assumed "FOO";
$a = \FOO; // fatal error, undefined namespace constant FOO
$a = Bar\FOO; // fatal error, undefined namespace constant bar\Bar\FOO
$a = \Bar\FOO; // fatal error, undefined namespace constant Bar\FOO
?>
      ]]>
     </programlisting>
    </example>
   </para>
  </sect2>
  <sect2 xml:id="language.namespaces.faq.builtinconst">
   <title>Cannot override special constants NULL, TRUE, FALSE, ZEND_THREAD_SAFE or ZEND_DEBUG_BUILD</title>
   <para>
    Any attempt to define a namespaced constant that is a special, built-in constant
    results in a fatal error
    <example>
     <title>Undefined constants</title>
     <programlisting role="php">
      <![CDATA[
<?php
namespace bar;
const NULL = 0; // fatal error;
const true = 'stupid'; // also fatal error;
// etc.
?>
      ]]>
     </programlisting>
    </example>
   </para>
  </sect2>
 </sect1>
</chapter>

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