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

<?xml version="1.0" encoding="iso-8859-1"?>
<!-- $Revision: 1.5 $ -->

 <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, of
    which contains the definition of the classes members and methods. Within
    each method, except for <link linkend="language.oop5.static">static</link>
    methods, a pseudo variable <varname>$this</varname> is available.
    <varname>$this</varname> is a  reference to the same instance that
    called the method.
   </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>
  </sect2>

  <sect2 id="language.oop5.basic.new">
   <title>new</title>
   <para>
    To create an instance of an object, 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.oop5.exceptions">exception</link> on error.
   </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 an object 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 new instance
    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.extend">
   <title>extend</title>
   <para>
    A class can inherit methods and members of another class by using the
    extend 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 overloaded, 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 overloaded
    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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