Implementing callback in PHP

In PHP, callback is a function object/reference with type callable. A callback/callable variable can act as a function, object method and a static class method. There are various ways to implement a callback. Some of them are discussed below:

Standard callback: In PHP, functions can be called using call_user_func() function where arguments is the string name of the function to be called.

Example:

<?php

// PHP program to illustrate working
// of a standard callback

// Function to print a string
function someFunction() {
    echo "Geeksforgeeks \n";
}

// Standard callback
call_user_func('someFunction');
?>

Output:

Geeksforgeeks

Static class method callback: Static class methods can be called by using call_user_func() where argument is an array containing the string name of class and the method inside it to be called.

Example:

<?php

// PHP program to illustrate working
// of a Static class method callback

// Sample class
class GFG {

    // Function used to print a string
    static function someFunction() {
        echo "Parent Geeksforgeeks \n";
    }
}

class Article extends GFG {

    // Function to print a string
    static function someFunction() {
        echo "Geeksforgeeks Article \n";
    }
}

// Static class method callback
call_user_func(array('Article', 'someFunction'));

call_user_func('Article::someFunction');

// Relative Static class method callback
call_user_func(array('Article', 'parent::someFunction'));
?>

Output:

Geeksforgeeks Article
Geeksforgeeks Article
Parent Geeksforgeeks

Object method callback: Object methods can be called by using call_user_func() where argument is an array containing the object variable and the string name of method to be called. Object method can also be called if they are made invokable using __invoke() function definition. In this case, argument to the call_user_func() function is the object variable itself.

Example:

<?php

// PHP program to illustrate working
// of a object method callback

// Sample class
class GFG {

    // Function to print a string
    static function someFunction() {
        echo "Geeksforgeeks \n";
    }

    // Function used to print a string
    public function __invoke() {
        echo "invoke Geeksforgeeks \n";
    }
}

// Class object
$obj = new GFG();

// Object method call
call_user_func(array($obj, 'someFunction'));

// Callable __invoke method object
call_user_func($obj);

?>

Output:

Geeksforgeeks
invoke Geeksforgeeks

Closure callback: Closure functions can be made callable by making standard calls or mapping closure function to array of valid arguments given to the closure function using array_map() function where arguments are the closure function and an array of its valid arguments.

Example:

<?php

// PHP program to illustrate working
// of a closure callback

// Closure to print a string
$print_function = function($string) {
    echo $string."\n";
};

// Array of strings
$string_array = array("Geeksforgeeks", "GFG", "Article");

// Callable closure
array_map($print_function, $string_array);

?>

Output:

Geeksforgeeks
GFG
Article