Callback Functions in PHP

A callback is a function that is passed as a parameter to another function.

Let's look at an example. Suppose we have a function that takes a number as its first parameter, and a callback as its second parameter:

<?php
	function func($num, $calb)
	{
		
	}
?>

Let's make it so that inside the function our callback is called for the passed number:

<?php
	function func($num, $calb)
	{
		echo $calb($num);
	}
?>

Now let's see what options there are for passing a callback to our function.

Option 1

Our callback can be a regular function:

<?php
	function calb($num) {
		return $num ** 2;
	}
?>

In this case, we pass the name of our function as the callback:

<?php
	func(3, 'calb');
?>

Inside the function func our callback will be called by name.

Our callback can be an anonymous function assigned to a variable:

<?php
	$calb = function($num) {
		return $num ** 2;
	};
?>

In this case, we pass the variable containing our function as a parameter:

<?php
	func(3, $calb);
?>

You can pass an anonymous function directly as a parameter:

<?php
	func(3, function($num) {
		return $num ** 2;
	});
?>

You can shorten the code by using an arrow function:

<?php
	func(3, fn($num) => $num ** 2);
?>

Given a function that takes an array and a callback as parameters, which will be applied to each element of the array:

<?php
	function func($arr, $calb)
	{
		$res = [];
		
		foreach ($arr as $elem) {
			$res[] = $calb($elem);
		}
		
		return $res;
	}
?>

Call this function, passing an array of numbers and a callback that squares the passed number as parameters.