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 10Note: 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.
← NULL
Pseudo-types and variables used in this documentation →
? 1997–2017 The PHP Documentation GroupLicensed under the Creative Commons Attribution License v3.0 or later.
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