Functions that can be passed as arguments to other functions or methods. 
Callback functions are commonly used in PHP for Event handling, sorting, filtering, and more.

Example:

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

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