6.11 PHP 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 1: PHP Callback Functions

Pass a callback to PHP's array_map() function to calculate the length of every string in an array:

<?php
function 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 2: PHP Callback Functions - Anonymous Function

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 3: PHP Callback Functions - Callbacks in User Defined Functions

Run a callback from a user-defined function:

<?php
function 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");
?>