PHP Callbacks

Last updated: April 6, 2026
Minimum PHP Version: PHP 5.0+

Overview

A callback function is a function which is passed as an argument into another function. Callbacks enable functional programming patterns and are fundamental to modern PHP development, especially with array functions and event systems.

When to Use

• Processing array elements with array_map(), array_filter(), array_reduce()
• Implementing event handlers and observers
• Creating higher-order functions
• Building flexible, reusable code
• Implementing design patterns like Strategy or Command

Basic Example

<?php
function processArray($callback, $array) {
    $result = [];
    foreach ($array as $item) {
        $result[] = $callback($item);
    }
    return $result;
}

function doubleValue($num) {
    return $num * 2;
}

$numbers = [1, 2, 3, 4];
$doubled = processArray("doubleValue", $numbers);
print_r($doubled);  // Output: Array ( [0] => 2 [1] => 4 [2] => 6 [3] => 8 )
?>

Advanced Example

<?php
// Using array_map with callbacks
$strings = ["apple", "orange", "banana"];
$lengths = array_map("strlen", $strings);

// Using anonymous functions
$numbers = [1, 2, 3, 4, 5];
$evens = array_filter($numbers, function($n) {
    return $n % 2 === 0;
});

// Arrow functions (PHP 7.4+)
$squared = array_map(fn($x) => $x ** 2, $numbers);
?>

Related Topics

• Arrow Functions
• Array Functions

PHP Version Support

Introduced: PHP 5.0
Minimum Required: PHP 5.0+
Anonymous Functions: PHP 5.3+
Arrow Functions: PHP 7.4+

See Also

• Official PHP Callbacks Documentation
• Array Functions

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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:

<?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:

<?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:

<?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");
?>

PHP callable Keyword

Use callable to require a callback function as an argument.

The callable keyword is used to force a function argument to be a reference to a function.

A callable can be one of the following:

• An anonymous function
• A string containing the name of a function
• An array describing a static class method
• An array describing an object method

<?php
function printFormatted(callable $format, $str) {
  echo $format($str);
  echo "<br>";
}

function exclaim($str) { return $str . "!"; }
printFormatted("exclaim", "Hello World");
?>

<?php
function printFormatted(callable $format, $str) {
  echo $format($str);
  echo "<br>";
}

class MyClass {
  public static function ask($str) {
    return $str . "?";
  }
  public function brackets($str) {
    return "[$str]";
  }
}

// An anonymous function
$func = function($str) { return substr($str, 0, 5); };
printFormatted($func , "Hello World");

// A string containing the name of a function
printFormatted("strtoupper", "Hello World");

// An array describing a static class method
printFormatted(["MyClass", "ask"], "Hello World");

// An array describing an object method
$obj = new MyClass();
printFormatted([$obj, "brackets"], "Hello World");
?>