php callback functions

Callback Functions

A callback function (often referred to as just "callback") is a function which is passed as an

argument into another function.

Any existing function can be used as a callback function. To use a function as a callback

function, pass a string containing the name of the function as the argument of another

function:

Example

Pass a callback to PHP's array_map() function to calculate the length of

every string in an array:

<?phpfunction my_callback($item) {  return strlen($item);}

$strings = ["apple", "orange",

"banana", "coconut"];$lengths =

array_map("my_callback", $strings);print_r($lengths);?>

Starting with version 7, PHP can pass anonymous functions as callback functions:

Example

Use an anonymous function as a callback for PHP's array_map() function:

<?php$strings = ["apple", "orange", "banana", "coconut"];$lengths =

array_map( function($item) { return strlen($item); } , $strings);

print_r($lengths);?>

Callbacks in User Defined Functions

User-defined functions and methods can also take callback functions as

arguments. To use callback functions inside a user-defined function or method,

call it by adding parentheses to the variable and pass arguments as with normal

functions:

Example

Run a callback from a user-defined function:

<?phpfunction exclaim($str) {  return $str . "! ";}

function ask($str) {  return $str . "? ";}function

printFormatted($str, $format) {  // Calling the $format callback

function  echo $format($str);}// Pass "exclaim" and "ask" as callback

functions to printFormatted()printFormatted("Hello world", "exclaim");

printFormatted("Hello world", "ask");?>