php-doc-en/language/oop5/basic.xml

<?xml version="1.0" encoding="utf-8"?>
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 <sect1 xml:id="language.oop5.basic" xmlns="http://docbook.org/ns/docbook">
  <title>The Basics</title>

  <sect2 xml:id="language.oop5.basic.class">
   <title>class</title>
   <para>
    Basic class definitions begin with the
    keyword <literal>class</literal>, followed by a class name,
    followed by a pair of curly braces which enclose the definitions
    of the properties and methods belonging to the class.
   </para>
   <para>
    The class name can be any valid label, provided it is not a
    PHP <link linkend="reserved">reserved word</link>. A valid class
    name starts with a letter or underscore, followed by any number of
    letters, numbers, or underscores. As a regular expression, it
    would be expressed thus:
    <literal>^[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*$</literal>.
   </para>
   <para>
    A class may contain its
    own <link linkend="language.oop5.constants">constants</link>, <link linkend="language.oop5.properties">variables</link>
    (called "properties"), and functions (called "methods").
   </para>
   <example>
    <title>Simple Class definition</title>
    <programlisting role="php">
<![CDATA[
<?php
class SimpleClass
{
    // property declaration
    public $var = 'a default value';

    // method declaration
    public function displayVar() {
        echo $this->var;
    }
}
?>
]]>
    </programlisting>
   </example>
   <para>
    The pseudo-variable <varname>$this</varname> is available when a
    method is called from within an object
    context. <varname>$this</varname> is a reference to the calling
    object (usually the object to which the method belongs, but
    possibly another object, if the method is called
    <link linkend="language.oop5.static">statically</link> from the context
    of a secondary object).
   </para>
   <para>
    <example xml:id="language.oop5.basic.class.this">
     <title>Some examples of the <varname>$this</varname> pseudo-variable</title>
     <programlisting role="php">
<![CDATA[
<?php
class A
{
    function foo()
    {
        if (isset($this)) {
            echo '$this is defined (';
            echo get_class($this);
            echo ")\n";
        } else {
            echo "\$this is not defined.\n";
        }
    }
}

class B
{
    function bar()
    {
        // Note: the next line will issue a warning if E_STRICT is enabled.
        A::foo();
    }
}

$a = new A();
$a->foo();

// Note: the next line will issue a warning if E_STRICT is enabled.
A::foo();
$b = new B();
$b->bar();

// Note: the next line will issue a warning if E_STRICT is enabled.
B::bar();
?>
]]>
     </programlisting>
     &example.outputs;
     <screen>
<![CDATA[
$this is defined (A)
$this is not defined.
$this is defined (B)
$this is not defined.
]]>
     </screen>
    </example>
   </para>
  </sect2>

  <sect2 xml:id="language.oop5.basic.new">
   <title>new</title>
   <para>
    To create an instance of a class, the <literal>new</literal> keyword must
    be used.  An object will always be created unless the object has a
    <link linkend="language.oop5.decon">constructor</link> defined that throws an
    <link linkend="language.exceptions">exception</link> on error. Classes
    should be defined before instantiation (and in some cases this is a
    requirement).
   </para>
   <para>
    If a <type>string</type> containing the name of a class is used with
    <literal>new</literal>, a new instance of that class will be created. If
    the class is in a namespace, its fully qualified name must be used when
    doing this.
   </para>
   <example>
    <title>Creating an instance</title>
    <programlisting role="php">
<![CDATA[
<?php
$instance = new SimpleClass();

// This can also be done with a variable:
$className = 'Foo';
$instance = new $className(); // Foo()
?>
]]>
    </programlisting>
   </example>
   <para>
    In the class context, it is possible to create a new object by
    <literal>new self</literal> and <literal>new parent</literal>.
   </para>
   <para>
    When assigning an already created instance of a class to a new variable, the new variable
    will access the same instance as the object that was assigned. This
    behaviour is the same when passing instances to a function. A copy
    of an already created object can be made by
    <link linkend="language.oop5.cloning">cloning</link> it.
   </para>
   <example>
    <title>Object Assignment</title>
    <programlisting role="php">
<![CDATA[
<?php

$instance = new SimpleClass();

$assigned   =  $instance;
$reference  =& $instance;

$instance->var = '$assigned will have this value';

$instance = null; // $instance and $reference become null

var_dump($instance);
var_dump($reference);
var_dump($assigned);
?>
]]>
    </programlisting>
    &example.outputs;
    <screen>
<![CDATA[
NULL
NULL
object(SimpleClass)#1 (1) {
   ["var"]=>
     string(30) "$assigned will have this value"
}
]]>
    </screen>
   </example>
   <para>
    PHP 5.3.0 introduced a couple of new ways to create instances of an
    object:
   </para>
   <example>
    <title>Creating new objects</title>
    <programlisting role="php">
<![CDATA[
<?php
class Test
{
    static public function getNew()
    {
        return new static;
    }
}

class Child extends Test
{}

$obj1 = new Test();
$obj2 = new $obj1;
var_dump($obj1 !== $obj2);

$obj3 = Test::getNew();
var_dump($obj3 instanceof Test);

$obj4 = Child::getNew();
var_dump($obj4 instanceof Child);
?>
]]>
    </programlisting>
    &example.outputs;
    <screen>
<![CDATA[
bool(true)
bool(true)
bool(true)
]]>
    </screen>
   </example>
  </sect2>

  <sect2 xml:id="language.oop5.basic.extends">
   <title>extends</title>
   <para>
    A class can inherit the methods and properties of another class by
    using the keyword <literal>extends</literal> in the class
    declaration. It is not possible to extend multiple classes; a
    class can only inherit from one base class.
   </para>
   <para>
    The inherited methods and properties can be overridden by
    redeclaring them with the same name defined in the parent
    class. However, if the parent class has defined a method
    as <link linkend="language.oop5.final">final</link>, that method
    may not be overridden.  It is possible to access the overridden
    methods or static properties by referencing them
    with <link linkend="language.oop5.paamayim-nekudotayim">parent::</link>.
   </para>
   <para>
    When overriding methods, the parameter signature should remain the same or
    PHP will generate an <constant>E_STRICT</constant> level error. This does
    not apply to the constructor, which allows overriding with different
    parameters.
   </para>
   <example>
    <title>Simple Class Inheritance</title>
    <programlisting role="php">
<![CDATA[
<?php
class ExtendClass extends SimpleClass
{
    // Redefine the parent method
    function displayVar()
    {
        echo "Extending class\n";
        parent::displayVar();
    }
}

$extended = new ExtendClass();
$extended->displayVar();
?>
]]>
    </programlisting>
    &example.outputs;
    <screen>
<![CDATA[
Extending class
a default value
]]>
    </screen>
   </example>
  </sect2>

  <sect2 xml:id="language.oop5.basic.class.class">
   <title>::class</title>

   <para>
    Since PHP 5.5, the <literal>class</literal> keyword is also used for class
    name resolution. You can get a string containing the fully qualified name
    of the <literal>ClassName</literal> class by using
    <literal>ClassName::class</literal>. This is particularly useful with
    <link linkend="language.namespaces">namespaced</link> classes.
   </para>
   <para>
    <example xml:id="language.oop5.basic.class.class.name">
     <title>Class name resolution</title>
     <programlisting role="php">
<![CDATA[
<?php
namespace NS {
    class ClassName {
    }
    
    echo ClassName::class;
}
?>
]]>
     </programlisting>
     &example.outputs;
     <screen>
<![CDATA[
NS\ClassName
]]>
     </screen>
    </example>
   </para>
  </sect2>
 </sect1>

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