Callbacks / Callables

Callbacks can be denoted by the callable type declaration.

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 either, passing the class name instead of an object at index 0, or passing 'ClassName::methodName'.

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

Note:

As of PHP 8.1.0, anonymous functions can also be created using the first class callable syntax.

Generally, any object implementing __invoke() 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
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!');
?>

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:

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.

add a note

User Contributed Notes 12 notes

up
277
andrewbessa at gmail dot com
12 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;
}

}
?>
up
193
computrius at gmail dot com
10 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
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208
steve at mrclay dot org
11 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.
up
189
Riikka K
9 years 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.
up
189
edanschwartz at gmail dot com
9 years 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`.
up
175
metamarkers at gmail dot com
11 years ago
you can pass an object as a callable if its class defines the __invoke() magic method..
up
113
mariano dot REMOVE dot perez dot rodriguez at gmail dot com
8 years 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.
up
9
InvisibleSmiley
3 years ago
If you pass a callable method to a function with a callable type declaration, the error message is misleading:

<?php
class X {
protected function
foo(): void {}
}

function
bar(callable $c) {}

$x = new X;
$c = [$x, 'foo'];
bar($c);
?>

Error message will be something like "Argument #1 ($c) must be of type callable, array given" while the actual problem here is only the visibility of method "foo". All you need to do is changing it to public (or use a different approach, e.g. with a Closure).
up
23
bradyn at NOSPAM dot bradynpoulsen dot com
8 years 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);
// => [..., ..., ...]
up
8
gulaschsuppe2 at gmail dot com
4 years ago
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

?>
up
3
pawel dot tadeusz dot niedzielski at gmail dot com
8 years 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();
?>
up
1
chris dot rutledge at gmail dot com
5 years ago
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)

?>
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