Callbacks

Callbacks are functions or methods which are used as parameters for other functions. Such functions are used to customize the calling function. For example, array_map(), which applies a callback to every element of an array, or usort(), which sorts an array using a function.

Callback is akin to a functioncall: it is actually hidden inside the caller function.

Callback may be build with on closures, arrow-functions, native PHP functions, methods and static methods. They also use string, such as strtolower, and array, such as [X::class, 'methodName'], or [$object, 'methodName'], syntax. In fact, anything that is_callable() may be used.

<?php

    $array = ['Damien', 'Dan', 'Daniel', 'Davey', 'David', 'Dennis', 'Derick', 'Dmitry', 'Dustin',];
    $array = array_map('strtolower', $array);

    //['damien', 'dan', 'daniel', 'davey', 'david', 'dennis', 'derick', 'dmitry', 'dustin',];

?>

Documentation

See also The Wonderful World of Callbacks, Using callbacks to prevent code duplication and How to Use PHP Callbacks: Examples and Guide.

Related : Callables, Closure, String, Arrow Functions, Functions, Method, Magic Methods, Array Callback, Listener, one