On the PHP-Coding-Practices site, there's a sort of case study posted showing how the author (Tim Koschuetzki) took a chunk of code that was slow at sorting an array and sped it up dramatically using the Quicksort Algorithm.

Today in the morning I had a very unusual programming job to do - or at least what is for me rather unusual. I was confronted with the question whether it is easy to implement the quicksort algorithmn to sort an array of arrays based on a key in the second-dimension of the array. Join me to find out.

The key to the transformation is an application of the Quicksort method that applies a filter to the elements in the array and places items to the right and left of the pivot point. To illustrate the point, he includes both the pre-quicksort code and post-quicksort.