PHP Callback Functions

A callback function is a function that passes as an argument into another function (commonly referred to as "callback").

Callback function Any existing function can be used. In order to use a function as a callback feature, use a string that contains another function's name :

Example 1 :- To calculate each string in a table, pass a callback to the PHP array_map() method :

<?php
function my_callback($item) {
  return strlen($item);
}
$strings = ["apple", "orange", "banana", "coconut"];
$lengths = array_map("my_callback", $strings);
print_r($lengths);
?>

Output :-

Example 2 :- As a callback to PHP's array_map() function, use an anonymous function:

<?php
$strings = ["apple", "orange", "banana", "coconut"];
$lengths = array_map( function($item) { return strlen($item); } , $strings);
print_r($lengths);
?>

Output :-

Callback functions can also be passed as arguments to user-defined functions and procedures. Call callback functions inside a user-defined function or method by enclosing the variable in parentheses and passing arguments as you would with regular functions :

Example :- Call a user-defined function with a callback :

<?php
function print1($str) {
  return $str . "! <br />";
}
function print2($str) {
  return $str . "? <br />";
}
function myPrint($str, $format) {
  // Calling the $format callback function
  echo $format($str);
}
// Pass "print1" and "print2" as callback functions to "myPrint".
myPrint("Hello world", "print1");
myPrint("Hello world", "print2");
?>

Output :-