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 "Geeksforneveropen \n"; }   // Standard callback call_user_func('someFunction'); ?> |
Geeksforneveropen
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 Geeksforneveropen \n";     } }   class Article extends GFG {       // Function to print a string     static function someFunction() {         echo "Geeksforneveropen 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')); ?> |
Geeksforneveropen Article Geeksforneveropen Article Parent Geeksforneveropen
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 "Geeksforneveropen \n";     }       // Function used to print a string     public function __invoke() {         echo "invoke Geeksforneveropen \n";     } }   // Class object $obj = new GFG();   // Object method call call_user_func(array($obj, 'someFunction'));   // Callable __invoke method object call_user_func($obj);   ?> |
Geeksforneveropen invoke Geeksforneveropen
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 $string_array = array("Geeksforneveropen", "GFG", "Article");   // Callable closure array_map($print_function, $string_array);   ?> |
Geeksforneveropen GFG Article
