Source of: /manual/en/language.types.callable.php
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<div id="language.types.callable" class="sect1">
<h2 class="title">Callbacks</h2>
<p class="para">
Callbacks can be denoted by <span class="type"><a href="language.types.callable.php" class="type callable">callable</a></span> type hint as of PHP 5.4.
This documentation used <span class="type"><a href="language.pseudo-types.php#language.types.callback" class="type callback">callback</a></span> type information for the same
purpose.
</p>
<p class="para">
Some functions like <span class="function"><a href="function.call-user-func.php" class="function">call_user_func()</a></span> or
<span class="function"><a href="function.usort.php" class="function">usort()</a></span> accept user-defined callback functions as a
parameter. Callback functions can not only be simple functions, but also
<span class="type"><a href="language.types.object.php" class="type object">object</a></span> methods, including static class methods.
</p>
<div class="sect2" id="language.types.callable.passing">
<h3 class="title">Passing</h3>
<p class="para">
A PHP function is passed by its name as a <span class="type"><a href="language.types.string.php" class="type string">string</a></span>. Any built-in
or user-defined function can be used, except language constructs such as:
<span class="function"><a href="function.array.php" class="function">array()</a></span>, <span class="function"><a href="function.echo.php" class="function">echo</a></span>,
<span class="function"><a href="function.empty.php" class="function">empty()</a></span>, <span class="function"><a href="function.eval.php" class="function">eval()</a></span>,
<span class="function"><a href="function.exit.php" class="function">exit()</a></span>, <span class="function"><a href="function.isset.php" class="function">isset()</a></span>,
<span class="function"><a href="function.list.php" class="function">list()</a></span>, <span class="function"><a href="function.print.php" class="function">print</a></span> or
<span class="function"><a href="function.unset.php" class="function">unset()</a></span>.
</p>
<p class="para">
A method of an instantiated <span class="type"><a href="language.types.object.php" class="type object">object</a></span> is passed as an
<span class="type"><a href="language.types.array.php" class="type array">array</a></span> containing an <span class="type"><a href="language.types.object.php" class="type object">object</a></span> at index 0 and the
method name at index 1.
</p>
<p class="para">
Static class methods can also be passed without instantiating an
<span class="type"><a href="language.types.object.php" class="type object">object</a></span> of that class by passing the class name instead of an
<span class="type"><a href="language.types.object.php" class="type object">object</a></span> at index 0.
As of PHP 5.2.3, it is also possible to pass
<em>'ClassName::methodName'</em>.
</p>
<p class="para">
Apart from common user-defined function, <span class="function"><a href="function.create-function.php" class="function">create_function()</a></span>
can also be used to create an anonymous callback function. As of PHP 5.3.0
it is possible to also pass a
<a href="functions.anonymous.php" class="link">closure</a> to a callback parameter.
</p>
<p class="para">
<div class="example" id="example-96">
<p><strong>Example #1
Callback function examples
</strong></p>
<div class="example-contents">
<div class="phpcode"><code><span style="color: #000000">
<span style="color: #0000BB"><?php <br /><br /></span><span style="color: #FF8000">// An example callback function<br /></span><span style="color: #007700">function </span><span style="color: #0000BB">my_callback_function</span><span style="color: #007700">() {<br /> echo </span><span style="color: #DD0000">'hello world!'</span><span style="color: #007700">;<br />}<br /><br /></span><span style="color: #FF8000">// An example callback method<br /></span><span style="color: #007700">class </span><span style="color: #0000BB">MyClass </span><span style="color: #007700">{<br /> static function </span><span style="color: #0000BB">myCallbackMethod</span><span style="color: #007700">() {<br /> echo </span><span style="color: #DD0000">'Hello World!'</span><span style="color: #007700">;<br /> }<br />}<br /><br /></span><span style="color: #FF8000">// Type 1: Simple callback<br /></span><span style="color: #0000BB">call_user_func</span><span style="color: #007700">(</span><span style="color: #DD0000">'my_callback_function'</span><span style="color: #007700">); <br /><br /></span><span style="color: #FF8000">// Type 2: Static class method call<br /></span><span style="color: #0000BB">call_user_func</span><span style="color: #007700">(array(</span><span style="color: #DD0000">'MyClass'</span><span style="color: #007700">, </span><span style="color: #DD0000">'myCallbackMethod'</span><span style="color: #007700">)); <br /><br /></span><span style="color: #FF8000">// Type 3: Object method call<br /></span><span style="color: #0000BB">$obj </span><span style="color: #007700">= new </span><span style="color: #0000BB">MyClass</span><span style="color: #007700">();<br /></span><span style="color: #0000BB">call_user_func</span><span style="color: #007700">(array(</span><span style="color: #0000BB">$obj</span><span style="color: #007700">, </span><span style="color: #DD0000">'myCallbackMethod'</span><span style="color: #007700">));<br /><br /></span><span style="color: #FF8000">// Type 4: Static class method call (As of PHP 5.2.3)<br /></span><span style="color: #0000BB">call_user_func</span><span style="color: #007700">(</span><span style="color: #DD0000">'MyClass::myCallbackMethod'</span><span style="color: #007700">);<br /><br /></span><span style="color: #FF8000">// Type 5: Relative static class method call (As of PHP 5.3.0)<br /></span><span style="color: #007700">class </span><span style="color: #0000BB">A </span><span style="color: #007700">{<br /> public static function </span><span style="color: #0000BB">who</span><span style="color: #007700">() {<br /> echo </span><span style="color: #DD0000">"A\n"</span><span style="color: #007700">;<br /> }<br />}<br /><br />class </span><span style="color: #0000BB">B </span><span style="color: #007700">extends </span><span style="color: #0000BB">A </span><span style="color: #007700">{<br /> public static function </span><span style="color: #0000BB">who</span><span style="color: #007700">() {<br /> echo </span><span style="color: #DD0000">"B\n"</span><span style="color: #007700">;<br /> }<br />}<br /><br /></span><span style="color: #0000BB">call_user_func</span><span style="color: #007700">(array(</span><span style="color: #DD0000">'B'</span><span style="color: #007700">, </span><span style="color: #DD0000">'parent::who'</span><span style="color: #007700">)); </span><span style="color: #FF8000">// A<br /></span><span style="color: #0000BB">?></span>
</span>
</code></div>
</div>
</div>
</p>
<p class="para">
<div class="example" id="example-97">
<p><strong>Example #2
Callback example using a Closure
</strong></p>
<div class="example-contents">
<div class="phpcode"><code><span style="color: #000000">
<span style="color: #0000BB"><?php<br /></span><span style="color: #FF8000">// Our closure<br /></span><span style="color: #0000BB">$double </span><span style="color: #007700">= function(</span><span style="color: #0000BB">$a</span><span style="color: #007700">) {<br /> return </span><span style="color: #0000BB">$a </span><span style="color: #007700">* </span><span style="color: #0000BB">2</span><span style="color: #007700">;<br />};<br /><br /></span><span style="color: #FF8000">// This is our range of numbers<br /></span><span style="color: #0000BB">$numbers </span><span style="color: #007700">= </span><span style="color: #0000BB">range</span><span style="color: #007700">(</span><span style="color: #0000BB">1</span><span style="color: #007700">, </span><span style="color: #0000BB">5</span><span style="color: #007700">);<br /><br /></span><span style="color: #FF8000">// Use the closure as a callback here to <br />// double the size of each element in our <br />// range<br /></span><span style="color: #0000BB">$new_numbers </span><span style="color: #007700">= </span><span style="color: #0000BB">array_map</span><span style="color: #007700">(</span><span style="color: #0000BB">$double</span><span style="color: #007700">, </span><span style="color: #0000BB">$numbers</span><span style="color: #007700">);<br /><br />print </span><span style="color: #0000BB">implode</span><span style="color: #007700">(</span><span style="color: #DD0000">' '</span><span style="color: #007700">, </span><span style="color: #0000BB">$new_numbers</span><span style="color: #007700">);<br /></span><span style="color: #0000BB">?></span>
</span>
</code></div>
</div>
<div class="example-contents"><p>The above example will output:</p></div>
<div class="example-contents screen">
<div class="cdata"><pre>
2 4 6 8 10
</pre></div>
</div>
</div>
</p>
<blockquote class="note"><p><strong class="note">Note</strong>:
<span class="simpara">
In PHP 4, it was necessary to use a reference to create a callback that
points to the actual <span class="type"><a href="language.types.object.php" class="type object">object</a></span>, and not a copy of it. For more
details, see
<a href="language.references.php" class="link">References Explained</a>.
</span>
</p></blockquote>
<blockquote class="note"><p><strong class="note">Note</strong>: <p class="para">Callbacks registered
with functions such as <span class="function"><a href="function.call-user-func.php" class="function">call_user_func()</a></span> and <span class="function"><a href="function.call-user-func-array.php" class="function">call_user_func_array()</a></span> will not be
called if there is an uncaught exception thrown in a previous callback.</p></p></blockquote>
</div>
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