Implementing callback in PHP

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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 "Geeksforzambiatek \n"; 
} 
  
// Standard callback 
call_user_func('someFunction'); 
?> 

Output:

Geeksforzambiatek

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 Geeksforzambiatek \n"; 
    } 
} 
  
class Article extends GFG { 
  
    // Function to print a string 
    static function someFunction() { 
        echo "Geeksforzambiatek 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:

Geeksforzambiatek Article
Geeksforzambiatek Article
Parent Geeksforzambiatek

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 "Geeksforzambiatek \n"; 
    } 
  
    // Function used to print a string 
    public function __invoke() { 
        echo "invoke Geeksforzambiatek \n"; 
    } 
} 
  
// Class object 
$obj = new GFG(); 
  
// Object method call 
call_user_func(array($obj, 'someFunction')); 
  
// Callable __invoke method object  
call_user_func($obj); 
  
?>  

Output:

Geeksforzambiatek
invoke Geeksforzambiatek

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("Geeksforzambiatek", "GFG", "Article"); 
  
// Callable closure 
array_map($print_function, $string_array); 
  
?> 