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

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<!-- $Revision: 1.19 $ -->

 <sect1 id="language.oop5.basic">
  <title>The Basics</title>

  <sect2 id="language.oop5.basic.class">
   <title>class</title>
   <para>
    Every class definition begins with the keyword class, followed by a class
    name, which can be any name that isn't a <link linkend="reserved">reserved</link>
    word in PHP. Followed  by a pair of curly braces,
    which contains the definition of the classes members and methods. A
    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 can be another object, if the method is called
    <link linkend="language.oop5.static">statically</link> from the context
    of a secondary object). This is illustrated in the following examples:
    <example>
     <title><varname>$this</varname> variable in object-oriented language</title>
     <programlisting role="php">
<![CDATA[
<?php
class A
{
    function foo()
    {
        if (isset($this)) {
            echo '$this is defined (';
            echo get_class($this);
            echo ")\n";
        } else {
            echo "\$this is not defined.\n";
        }
    }
}

class B
{
    function bar()
    {
        A::foo();
    }
}

$a = new A();
$a->foo();
A::foo();
$b = new B();
$b->bar();
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>
   <example>
    <title>Simple Class definition</title>
    <programlisting role="php">
<![CDATA[
<?php
class SimpleClass
{
    // member declaration
    public $var = 'a default value';

    // method declaration
    public function displayVar() {
        echo $this->var;
    }
}
?>
]]>
    </programlisting>
   </example>

   <para>
    The default value must be a constant expression, not (for example) a
    variable, a class member or a function call.
    <example>
     <title>Class members' default value</title>
     <programlisting role="php">
<![CDATA[
<?php
class SimpleClass
{
    // invalid member declarations:
    public $var1 = 'hello '.'world';
    public $var2 = <<<EOD
hello world
EOD;
    public $var3 = 1+2;
    public $var4 = self::myStaticMethod();
    public $var5 = $myVar;

    // valid member declarations:
    public $var6 = myConstant;
    public $var7 = self::classConstant;
    public $var8 = array(true, false);
    
    
}
?>
]]>
     </programlisting>   
    </example>
   </para>
   <note>
    <para>
    There are some nice functions to handle classes and objects. You might want
    to take a look at the <link linkend="ref.classobj">Class/Object
    Functions</link>.
    </para>
   </note>
  </sect2>

  <sect2 id="language.oop5.basic.new">
   <title>new</title>
   <para>
    To create an instance of a class, a new object must be created and
    assigned to a variable.  An object will always be assigned when
    creating a new object 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>
   <example>
    <title>Creating an instance</title>
    <programlisting role="php">
<![CDATA[
<?php
$instance = new SimpleClass();
?>
]]>
    </programlisting>
   </example>
   <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
$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>
  </sect2>

  <sect2 id="language.oop5.basic.extends">
   <title>extends</title>
   <para>
    A class can inherit methods and members of another class by using the
    extends keyword in the declaration. It is not possible to extend multiple
    classes, a class can only inherit one base class.
   </para>
   <para>
    The inherited methods and members can be overridden, unless the parent
    class has defined a method as <link linkend="language.oop5.final">final</link>,
    by redeclaring them within the same name defined in the parent class.
    It is possible to access the overrided method or members by
    referencing them with <link linkend="language.oop5.paamayim-nekudotayim">parent::</link>
   </para>
   <example>
    <title>Simple Class Inherintance</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>

 </sect1>

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