callable.xml

<?xml version="1.0" encoding="utf-8"?>
<!-- $Revision$ -->
<sect1 xml:id="language.types.callable">
 <title>Callbacks / Callables</title>

 <para>
  Callbacks can be denoted by the <type>callable</type> type declaration.
 </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. Passing protected and private methods
   with the <type>callable</type> syntax is allowed.
  </para>

  <note>
   <para>
    If a callable of a protected or a private method is invoked from outside the class
    with no visibility of said methods (e.g. being called from a POSIX signal handler),
    a runtime error is thrown because the callable is invalid at the time of invocation.
   </para>
  </note>

  <para>
   Static class methods can also be passed without instantiating an
   <type>object</type> of that class by either, passing the class name
   instead of an <type>object</type> at index 0, or passing
   <literal>'ClassName::methodName'</literal>.
  </para>

  <para>
   Apart from common user-defined function,
   <link linkend="functions.anonymous">anonymous functions</link> and
   <link linkend="functions.arrow">arrow functions</link> can also be
   passed to a callback parameter.
  </para>

  <note>
   <para>
    As of PHP 8.1.0, anonymous functions can also be created using the <link linkend="functions.first_class_callable_syntax">first class callable syntax</link>.
   </para>
  </note>

  <para>
   Generally, any object implementing <link linkend="object.invoke">__invoke()</link> can also
   be passed 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
call_user_func('MyClass::myCallbackMethod');

// Type 5: Relative static class method call
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, deprecated as of PHP 8.2.0

// Type 6: Objects implementing __invoke can be used as callables
class C {
    public function __invoke($name) {
        echo 'Hello ', $name, "\n";
    }
}

$c = new C();
call_user_func($c, 'PHP!');
?>
]]>
    </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>

  <para>
   <example>
    <title>
     Manually invoking callbacks
    </title>
   <programlisting role="php">
<![CDATA[
<?php

// define a callback handler that executes the callable (no parameters)
function invokeCallback(callable $theCallback) {
    $theCallback();
}

// prints "42"
invokeCallback(function () {
    echo "42" . PHP_EOL;
});

// define a callback handler that executes the callable with a parameter
function processNumber(callable $processor, int $theNumber) {
    // process a number, and print its result
    $result = $processor($theNumber);
    echo $result . PHP_EOL;
}

// prints "72"
processNumber(fn ($x) => $x * 3, 24);

// define a class implementing the __invoke method
class NumberTripler
{
    public function __invoke($x)
    {
        return $x * 3;
    }
}

// prints "15"
$tripler = new NumberTripler();
processNumber($tripler, 5);

// define a class with a private instance method, to be called with the callable syntax
class HiddenFactory
{
    private function hiddenWork()
    {
        echo "256" . PHP_EOL;
    }

    public function getProof()
    {
        return Closure::fromCallable([$this, 'hiddenWork']);
    }
}

// prints "256"
$factory = new HiddenFactory();
$theProof = $factory->getProof();
invokeCallback($theProof);
?>
    </programlisting>
    &example.outputs;
    <screen>
<![CDATA[
42
72
15
256
]]>
    </screen>
   </example>
  </para>

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

</sect1>

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