The preg_replace_callback() function is an under-utilized and little documented PHP function that you will find useful and powerful in many situations, especially if you have been actively using preg_match() and preg_replace(). In this article, you will find a clear explanation of what this function does, together with carefully crafted examples that illustrate some of its uses.

They show how to use it in a few different situations - inside conditional statements, for loops, in a sequence and inside of a class to manipulate matched strings with external functions (both native to PHP and user-defined).