Callback / Callable 类型

自 PHP 5.4 起可用 callable 类型指定回调类型 callback。本文档基于同样理由使用 callback 类型信息。

一些函数如 call_user_func() 或 usort() 可以接受用户自定义的回调函数作为参数。回调函数不止可以是简单函数，还可以是对象的方法，包括静态类方法。

传递

PHP是将函数以string形式传递的。 可以使用任何内置或用户自定义函数，但除了语言结构例如：array()，echo，empty()，eval()，exit()，isset()，list()，print 或 unset()。

一个已实例化的 object 的方法被作为 array 传递，下标 0 包含该 object，下标 1 包含方法名。 在同一个类里可以访问 protected 和 private 方法。

静态类方法也可不经实例化该类的对象而传递，只要在下标 0 中包含类名而不是对象。自 PHP 5.2.3 起，也可以传递 'ClassName::methodName'。

除了普通的用户自定义函数外，也可传递 匿名函数 给回调参数。

Example #1 回调函数示例

<?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 使用 Closure 的示例

<?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);
?>

以上例程会输出：

2 4 6 8 10

Note:

在函数中注册有多个回调内容时(如使用 call_user_func() 与 call_user_func_array())，如在前一个回调中有未捕获的异常，其后的将不再被调用。

User Contributed Notes

gulaschsuppe2 at gmail dot com 21-Jul-2019 01:53
I tried many possible ways of calling functions by function name directly and assigned to a variable on 3v4l. Not mentioned yet, it is possible to use an array as a caller, at least since PHP 7.1.25. The following script contains all the information I gained:

<?php

// Call function via function name:
    // Basics:
        // A function can also be called by using its string name:
        function callbackFunc() {
            echo 'Hello World';
        }

        'callbackFunc'(); // Hello World
                           
        // A function can also be called if its name is assigned to a variable:
            function callbackFunc() {
                echo 'Hello World';
            }

            $funcName = 'callbackFunc';
            $funcName(); // Hello World

    // Static class method:
        // It is also possible to call a public static class method via 'ClassName::functioName' notation:
            class A {
                public static function callbackMethod() {
                    echo "Hello World\n";
                }
            }
            'A::callbackMethod'(); // Hello World

            $funcName = 'A::callbackMethod';
            $funcName(); // Hello World

    // Non static class method:
        // It is also possible to call non static class methods by creating an array which first element is the object the method should be called on and the second element is the non static method to be called. The array can directly be used as a caller:
            class A {
                private $prop = "Hello World\n";

                public function callbackMethod() {
                    echo $this->prop;
                }
            }

            $a = new A;
            [$a, 'callbackMethod']();
            $funcCallArr = [$a, 'callbackMethod'];
            $funcCallArr();

        // Of course this also works inside the class with '$this':
            class A {
                private function privCallback() {
                    echo 'Private';
                }

                public function privCallbackCaller($funcName) {
                    [$this, $funcName]();
                }
            }

            (new A)->privCallbackCaller('privCallback'); // Private

?>
chris dot rutledge at gmail dot com 18-Dec-2018 10:12
Having read this line in the manual above,

"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."

I decided to do some testing to see if I could access private methods using the call_user_func methods. Thankfully not, but for completeness here is my test which also covers using static and object contexts

<?php
class foo {
   
    public static $isInstance = false;
   
    public function __construct() {
        self::$isInstance = true;
    }

    public function bar() {
        var_dump(self::$isInstance);
        echo __METHOD__;
    }
   
    private function baz() {
        var_dump(self::$isInstance);
        echo __METHOD__;
    }
   
    public function qux() {
        $this->baz();
    }
   
    public function quux() {
        self::baz();
    }
}

call_user_func(['foo','bar']);    //fase, foo:bar

call_user_func(['foo','baz']);  //warning, cannot access private method

call_user_func(['foo','quux']); //false, foo::baz

call_user_func(['foo','qux']);  //fatal, Using $this when not in object context

$foo = new foo;

call_user_func([$foo,'bar']);    //true, foo::bar
call_user_func([$foo,'baz']);    //warning, cannot access private method
call_user_func([$foo,'qux']);    //true, foo::baz

call_user_func(['foo','bar']);  //true, foo::bar (static call, yet $isInstance is true)

?>
Dan J 20-Feb-2018 05:59
You can avoid repeating a long namespace for classes in callable arrays by making use of the "use" operator and the special "::class" constant.

Documentation of use operator:
http://php.net/manual/en/language.namespaces.importing.php

Documentation of ::class constant:
http://php.net/manual/en/language.oop5.constants.php

<?php
// Library file with namespace My\Library\Namespace
require 'MyLibrary.php';

// Alias for SortingClass
use \My\Library\Namespace\SortingClass;

// Callable array referring to SortingClass::SortFunction
$callable = [SortingClass::class, 'SortFunction'];

$values = [3, 1, 2];
usort($values, $callable);
Daniel Klein 13-Sep-2017 07:26
You can use "self::method_name", "static::method_name" and "parent::method_name" in callables:

<?php
class StaticCallable {
    public static function foo($values) {
        return array_map('self::bar', $values);
    }

    public static function bar($value) {
        return "{$value}: 42";
    }

    public static function baz($values) {
        return array_map('static::qux', $values);
    }

    public static function qux($value) {
        return "{$value}: 123";
    }
}

class StaticExtension extends StaticCallable {
    public static function bar($value) {
        return "{$value}: Marvin the Paranoid Android";
    }

    public static function qux($value) {
        return "{$value}: Zaphod Beeblebrox";
    }
}

print_r(StaticCallable::foo([1, 2, 3]));
print_r(StaticExtension::foo([1, 2, 3]));

print_r(StaticCallable::baz([1, 2, 3]));
print_r(StaticExtension::baz([1, 2, 3]));
?>

Results:
Array
(
    [0] => 1: 42
    [1] => 2: 42
    [2] => 3: 42
)
Array
(
    [0] => 1: 42
    [1] => 2: 42
    [2] => 3: 42
)
Array
(
    [0] => 1: 123
    [1] => 2: 123
    [2] => 3: 123
)
Array
(
    [0] => 1: Zaphod Beeblebrox
    [1] => 2: Zaphod Beeblebrox
    [2] => 3: Zaphod Beeblebrox
)

"self::" uses the same class as the called method, "static::" uses the same class as the called class, and "parent::" (not shown) uses the parent class, or generates a warning if there is no parent.
whysteepy at gmail dot com 18-Apr-2017 06:34
Another Appearance of Callbacks! Here is one way of them - methods of an instantiated object can be callable and implemented as variable functions without php's default functions that can call user-defined callback functions.

class Test {
    protected $items = array();

    public function __construct()
    {
        $this->items[] = array($this, 'callBackOne');
        $this->items[] = array($this, 'callBackTwo');
    }

    public function callBackOne()
    {
        echo __METHOD__ . ' has been called as a callback.';
    }

    public function callBackTwo()
    {
        echo __METHOD__ . ' has been called as a callback.';
    }   

    public function getItems()
    {
        return $this->items;
    }
}

$o = new Test();
$itemLists = $o->getItems();

foreach ($itemLists as $itemList) {

        // call each one as a variable function
        echo '<pre>';
        print_r($itemList());
        echo '</pre>';
}

// Outputs the following
// Test::callBackOne has been called as a callback.

// Test::callBackTwo has been called as a callback.
bradyn at NOSPAM dot bradynpoulsen dot com 14-Apr-2016 01:22
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 12-Apr-2016 02:11
@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();
?>
mariano dot REMOVE dot perez dot rodriguez at gmail dot com 21-Sep-2015 07:45
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.
Riikka K 11-May-2015 01:36
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.
edanschwartz at gmail dot com 02-Feb-2015 06:13
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`.
Yzmir Ramirez 15-Apr-2014 11:40
> 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.
computrius at gmail dot com 11-Oct-2013 04:38
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
metamarkers at gmail dot com 03-Jun-2013 07:34
you can pass an object as a callable if its class defines the __invoke() magic method..
steve at mrclay dot org 17-Sep-2012 09:00
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.
andrewbessa at gmail dot com 19-Jun-2012 03:16
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;
    }

}
?>
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