Callbacks / Callables

Callbacks can be denoted by callable type hint as of PHP 5.4. This documentation used callback type information for the same purpose.

Some functions like call_user_func() or usort() accept user-defined callback functions as a parameter. Callback functions can not only be simple functions, but also object methods, including static class methods.

Passing

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

A method of an instantiated object is passed as an array containing an object at index 0 and the method name at index 1. Accessing protected and private methods from within a class is allowed.

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

Apart from common user-defined function, anonymous functions can also be passed to a callback parameter.

Example #1 Callback function examples


<?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

// Type 6: Objects implementing __invoke can be used as callables (since PHP 5.3)
class C {
    public function __invoke($name) {
        echo 'Hello ', $name, "\n";
    }
}

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

Example #2 Callback example using a Closure


<?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);
?>

The above example will output:

2 4 6 8 10

Note: In PHP 4, it was necessary to use a reference to create a callback that points to the actual object, and not a copy of it. For more details, see References Explained.

Note:
Callbacks registered with functions such as call_user_func() and call_user_func_array() will not be called if there is an uncaught exception thrown in a previous callback.

User Contributed Notes

edanschwartz at gmail dot com

1 year ago


You can use 'self::methodName' as a callable, but this is dangerous. Consider this example:

<?php
class Foo {
    public static function doAwesomeThings() {
        FunctionCaller::callIt('self::someAwesomeMethod');
    }

    public static function someAwesomeMethod() {
        // fantastic code goes here.
    }
}

class FunctionCaller {
    public static function callIt(callable $func) {
        call_user_func($func);
    }
}

Foo::doAwesomeThings();
?>

This results in an error:
Warning: class 'FunctionCaller' does not have a method 'someAwesomeMethod'.

For this reason you should always use the full class name:
<?php
FunctionCaller::callIt('Foo::someAwesomeMethod');
?>

I believe this is because there is no way for FunctionCaller to know that the string 'self' at one point referred to to `Foo`.

computrius at gmail dot com

2 years ago


When specifying a call back in array notation (ie. array($this, "myfunc") ) the method can be private if called from inside the class, but if you call it from outside you'll get a warning:



<?php



class mc {

   public function go(array $arr) {

       array_walk($arr, array($this, "walkIt"));

   }



   private function walkIt($val) {

       echo $val . "<br />";

   }



    public function export() {

        return array($this, 'walkIt');

    }

}



$data = array(1,2,3,4);



$m = new mc;

$m->go($data); // valid



array_walk($data, $m->export()); // will generate warning



?>



Output:

1<br />2<br />3<br />4<br />

Warning: array_walk() expects parameter 2 to be a valid callback, cannot access private method mc::walkIt() in /in/tfh7f on line 22

steve at mrclay dot org

3 years ago


Performance note: The callable type hint, like is_callable(), will trigger an autoload of the class if the value looks like a static method callback.

Riikka K

11 months ago


A note on differences when calling callbacks as "variable functions" without the use of call_user_func() (e.g. "<?php $callback = 'printf'; $callback('Hello World!') ?>"):

- Using the name of a function as string has worked since at least 4.3.0
- Calling anonymous functions and invokable objects has worked since 5.3.0
- Using the array structure [$object, 'method'] has worked since 5.4.0

Note, however, that the following are not supported when calling callbacks as variable functions, even though they are supported by call_user_func():

- Calling static class methods via strings such as 'foo::doStuff'
- Calling parent method using the [$object, 'parent::method'] array structure

All of these cases are correctly recognized as callbacks by the 'callable' type hint, however. Thus, the following code will produce an error "Fatal error: Call to undefined function foo::doStuff() in /tmp/code.php on line 4":

<?php
class foo {
    static function callIt(callable $callback) {
        $callback();
    }
    
    static function doStuff() {
        echo "Hello World!";
    }
}

foo::callIt('foo::doStuff');
?>

The code would work fine, if we replaced the '$callback()' with 'call_user_func($callback)' or if we used the array ['foo', 'doStuff'] as the callback instead.

andrewbessa at gmail dot com

3 years ago


You can also use the $this variable to specify a callback:



<?php

class MyClass {



    public $property = 'Hello World!';



    public function MyMethod()

    {

        call_user_func(array($this, 'myCallbackMethod'));

    }



    public function MyCallbackMethod()

    {

        echo $this->property;

    }



}

?>

metamarkers at gmail dot com

2 years ago


you can pass an object as a callable if its class defines the __invoke() magic method..

Yzmir Ramirez

2 years ago


> As of PHP 5.2.3, it is also possible to pass 'ClassName::methodName'

You can also use 'self::methodName'.  This works in PHP 5.2.12 for me.

mariano dot REMOVE dot perez dot rodriguez at gmail dot com

7 months ago


I needed a function that would determine the type of callable being passed, and, eventually,
normalized it to some extent. Here's what I came up with:

<?php

/**
 * The callable types and normalizations are given in the table below:
 *
 *  Callable                        | Normalization                   | Type
 * ---------------------------------+---------------------------------+--------------
 *  function (...) use (...) {...}  | function (...) use (...) {...}  | 'closure'
 *  $object                         | $object                         | 'invocable'
 *  "function"                      | "function"                      | 'function'
 *  "class::method"                 | ["class", "method"]             | 'static'
 *  ["class", "parent::method"]     | ["parent of class", "method"]   | 'static'
 *  ["class", "self::method"]       | ["class", "method"]             | 'static'
 *  ["class", "method"]             | ["class", "method"]             | 'static'
 *  [$object, "parent::method"]     | [$object, "parent::method"]     | 'object'
 *  [$object, "self::method"]       | [$object, "method"]             | 'object'
 *  [$object, "method"]             | [$object, "method"]             | 'object'
 * ---------------------------------+---------------------------------+--------------
 *  other callable                  | idem                            | 'unknown'
 * ---------------------------------+---------------------------------+--------------
 *  not a callable                  | null                            | false
 *
 * If the "strict" parameter is set to true, additional checks are
 * performed, in particular:
 *  - when a callable string of the form "class::method" or a callable array
 *    of the form ["class", "method"] is given, the method must be a static one,
 *  - when a callable array of the form [$object, "method"] is given, the
 *    method must be a non-static one.
 *
 */
function callableType($callable, $strict = true, callable& $norm = null) {
  if (!is_callable($callable)) {
    switch (true) {
      case is_object($callable):
        $norm = $callable;
        return 'Closure' === get_class($callable) ? 'closure' : 'invocable';
      case is_string($callable):
        $m    = null;
        if (preg_match('~^(?<class>[a-z_][a-z0-9_]*)::(?<method>[a-z_][a-z0-9_]*)$~i', $callable, $m)) {
          list($left, $right) = [$m['class'], $m['method']];
          if (!$strict || (new \ReflectionMethod($left, $right))->isStatic()) {
            $norm = [$left, $right];
            return 'static';
          }
        } else {
          $norm = $callable;
          return 'function';
        }
        break;
      case is_array($callable):
        $m = null;
        if (preg_match('~^(:?(?<reference>self|parent)::)?(?<method>[a-z_][a-z0-9_]*)$~i', $callable[1], $m)) {
          if (is_string($callable[0])) {
            if ('parent' === strtolower($m['reference'])) {
              list($left, $right) = [get_parent_class($callable[0]), $m['method']];
            } else {
              list($left, $right) = [$callable[0], $m['method']];
            }
            if (!$strict || (new \ReflectionMethod($left, $right))->isStatic()) {
              $norm = [$left, $right];
              return 'static';
            }
          } else {
            if ('self' === strtolower($m['reference'])) {
              list($left, $right) = [$callable[0], $m['method']];
            } else {
              list($left, $right) = $callable;
            }
            if (!$strict || !(new \ReflectionMethod($left, $right))->isStatic()) {
              $norm = [$left, $right];
              return 'object';
            }
          }
        }
        break;
    }
    $norm = $callable;
    return 'unknown';
  }
  $norm = null;
  return false;
}

?>

Hope someone else finds it useful.

bradyn at NOSPAM dot bradynpoulsen dot com

18 days ago


When trying to make a callable from a function name located in a namespace, you MUST give the fully qualified function name (regardless of the current namespace or use statements).

<?php

namespace MyNamespace;

function doSomethingFancy($arg1)
{
    // do something...
}

$values = [1, 2, 3];

array_map('doSomethingFancy', $values);
// array_map() expects parameter 1 to be a valid callback, function 'doSomethingFancy' not found or invalid function name

array_map('MyNamespace\doSomethingFancy', $values);
// => [..., ..., ...]

pawel dot tadeusz dot niedzielski at gmail dot com

19 days ago


@edanschwartz at gmail dot com

You can use ::class property to always indicate the class you're in when using static methods:

<?php
class Foo {
    public static function doAwesomeThings() {
        FunctionCaller::callIt(self::class . '::someAwesomeMethod');
    }

    public static function someAwesomeMethod() {
        // fantastic code goes here.
    }
}

class FunctionCaller {
    public static function callIt(callable $func) {
        call_user_func($func);
    }
}

Foo::doAwesomeThings();
?>