php-doc-en/reference/overload/reference.xml

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 <reference id="ref.overload">
  <title>Object property and method call overloading</title>
  <titleabbrev>Object overloading</titleabbrev>

  <partintro>

   <section id="overload.intro">
    &reftitle.intro;
    <para>
     The purpose of this extension is to allow overloading of object
     property access and method calls. Only one function is defined
     in this extension, <function>overload</function> which
     takes the name of the class that should have this functionality
     enabled. The class named has to define appropriate methods if
     it wants to have this functionality: <literal>__get()</literal>,
     <literal>__set()</literal> and <literal>__call()</literal>
     respectively for getting/setting a property, or calling a method.
     This way overloading can be selective. Inside these handler
     functions the overloading is disabled so you can access object
     properties normally.
    </para>
    &warn.experimental;
   </section>

   <section id="overload.requirements">
    &reftitle.required;
    &no.requirement;
   </section>

   &reference.overload.configure;

   <section id="overload.configuration">
    &reftitle.runtime;
    &no.config;
   </section>

   <section id="overload.resources">
    &reftitle.resources;
    &no.resource;
   </section>

   <section id="overload.constants">
    &reftitle.constants;
    &no.constants;
   </section>

   <section id="overload.examples">
    &reftitle.examples;
    <para>
     Some simple examples on using the <function>overload</function>
     function:
     <example>
      <title>Overloading a PHP class</title>
      <programlisting role="php">
<![CDATA[
<?php

class OO
{
    var $a = 111;
    var $elem = array('b' => 9, 'c' => 42);

    // Callback method for getting a property
    function __get($prop_name, &$prop_value)
    {
        if (isset($this->elem[$prop_name])) {
            $prop_value = $this->elem[$prop_name];
            return true;
        } else {
            return false;
        }
    }

    // Callback method for setting a property
    function __set($prop_name, $prop_value)
    {
        $this->elem[$prop_name] = $prop_value;
        return true;
    }
}

// Here we overload the OO object
overload('OO');

$o = new OO;
print "\$o->a: $o->a\n"; // print: $o->a:
print "\$o->b: $o->b\n"; // print: $o->b: 9
print "\$o->c: $o->c\n"; // print: $o->c: 42
print "\$o->d: $o->d\n"; // print: $o->d:

// add a new item to the $elem array in OO
$o->x = 56; 

// instantiate stdclass (it is built-in in PHP 4)
// $val is not overloaded!
$val = new stdclass;
$val->prop = 555;

// Set "a" to be an array with the $val object in it
// But __set() will put this in the $elem array
$o->a = array($val);
var_dump($o->a[0]->prop);

?>
]]>
      </programlisting>
     </example>
    </para>
    <warning>
     <para>
      As this is an experimental extension, not all things
      work. There is no <literal>__call()</literal> support
      currently, you can only overload the get and set
      operations for properties. You cannot invoke the
      original overloading handlers of the class, and
      <literal>__set()</literal> only works to one level
      of property access.
     </para>
    </warning>
   </section>
  </partintro>

&reference.overload.functions;

 </reference>

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