php-doc-en/language/types/callable.xml

<?xml version="1.0" encoding="utf-8"?>
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<sect1 xml:id="language.types.callable">
 <title>Callbacks</title>
 
 <para>
  Callbacks can be denoted by <type>callable</type> type hint as of PHP 5.4.
  This documentation used <type>callback</type> type information for the same
  purpose.
 </para>

 <para>
  Some functions like <function>call_user_func</function> or
  <function>usort</function> accept user-defined callback functions as a
  parameter. Callback functions can not only be simple functions, but also
  <type>object</type> methods, including static class methods.
 </para>

 <sect2 xml:id="language.types.callable.passing">
  <title>Passing</title>

  <para>
   A PHP function is passed by its name as a <type>string</type>. Any built-in
   or user-defined function can be used, except language constructs such as:
   <function>array</function>, <function>echo</function>,
   <function>empty</function>, <function>eval</function>, 
   <function>exit</function>, <function>isset</function>, 
   <function>list</function>, <function>print</function> or
   <function>unset</function>.
  </para>

  <para>
   A method of an instantiated <type>object</type> is passed as an
   <type>array</type> containing an <type>object</type> at index 0 and the
   method name at index 1.
  </para>

  <para>
   Static class methods can also be passed without instantiating an
   <type>object</type> of that class by passing the class name instead of an
   <type>object</type> at index 0.
   As of PHP 5.2.3, it is also possible to pass
   <literal>'ClassName::methodName'</literal>.
  </para>

  <para>
   Apart from common user-defined function, <function>create_function</function>
   can also be used to create an anonymous callback function. As of PHP 5.3.0 
   it is possible to also pass a
   <link linkend="functions.anonymous">closure</link> to a callback parameter.
  </para>

  <para>
   <example>
    <title>
     Callback function examples
    </title>
    <programlisting role="php">
<![CDATA[
<?php 

// An example callback function
function my_callback_function() {
    echo 'hello world!';
}

// An example callback method
class MyClass {
    static function myCallbackMethod() {
        echo 'Hello World!';
    }
}

// Type 1: Simple callback
call_user_func('my_callback_function'); 

// Type 2: Static class method call
call_user_func(array('MyClass', 'myCallbackMethod')); 

// Type 3: Object method call
$obj = new MyClass();
call_user_func(array($obj, 'myCallbackMethod'));

// Type 4: Static class method call (As of PHP 5.2.3)
call_user_func('MyClass::myCallbackMethod');

// Type 5: Relative static class method call (As of PHP 5.3.0)
class A {
    public static function who() {
        echo "A\n";
    }
}

class B extends A {
    public static function who() {
        echo "B\n";
    }
}

call_user_func(array('B', 'parent::who')); // A
?>
]]>
    </programlisting>
   </example>
  </para>
  <para>
   <example>
    <title>
     Callback example using a Closure
    </title>
   <programlisting role="php">
<![CDATA[
<?php
// Our closure
$double = function($a) {
    return $a * 2;
};

// This is our range of numbers
$numbers = range(1, 5);

// Use the closure as a callback here to 
// double the size of each element in our 
// range
$new_numbers = array_map($double, $numbers);

print implode(' ', $new_numbers);
?>
]]>
    </programlisting>
    &example.outputs;
    <screen>
<![CDATA[
2 4 6 8 10
]]>
    </screen>
   </example>
  </para>

  <note>
   <simpara>
    In PHP 4, it was necessary to use a reference to create a callback that
    points to the actual <type>object</type>, and not a copy of it. For more
    details, see
    <link linkend="language.references">References Explained</link>.
   </simpara>
  </note>

  &note.func-callback-exceptions;
 </sect2>

</sect1>
 
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