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

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

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

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

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

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

Option 2

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

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

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

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

Option 3

You can pass an anonymous function directly as a parameter:

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

Option 4

You can shorten the code by using an arrow function:

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

Practical Tasks

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