PHP callbacks

First what is a callback?

In computer programming, a callback is a piece of executable code that is passed as an argument to other code, which is expected to call back (execute) the argument at some convenient time.

definition from wikipedia

Clearly said.

Callbacks can be declared using a user-defined function including anonymous function or an array of Object at index[0] and a method at index [1]. e.g.

$function_callback = function($x) {
	return $x * 2;
}

$object_method_callback = array($object, 'method');
// -- or -- for static methods
// array('MyClass', 'method')
// 'MyClass::method'

Callback is denoted by callable type hint

This is usefull when using built-in PHP functions that accepts callbacks as one of its parameter. Functions such as array_filter, usort, call_user_func, call_user_func_array, forward_static_call, forward_static_call, etc.

Example using array_filter to filter elements in an array.

Syntax:

/**
 * @param $array     array  	The array to filter
 * @param $callback  callable	The callback  
 * @returns array
 */
array_filter ( array $array [, callable $callback ] );

Below we use anonymouse function for the callback.

// create array with values from 1 to 20
$array = range(1, 20);

$new_arr = array_filter($array, function($value) {
	return ($value % 2) == 0;
});

// prints new set of array which is divisible by 2
print_r($new_arr);

...is accomplished with the same code below by passing the name of callback function as second argument.

function divisibleByTwo($value) {
	return ($value % 2) == 0;
}

$array = range(1, 20);

$new_arr = array_filter($array, 'divisibleByTwo');

print_r($new_arr);

How about callback using an array of Class and method? Example below.

class Number {
	public static function divisibleByTwo($value) {
		return ($value % 2) == 0;
	}
}
	
$array = range(1, 20);

$new_arr = array_filter($array, array('Number', 'divisibleByTwo'));

print_r($new_arr);

Callback is such a powerfull tool, and as always learning the basics would give you a good foundation.