How to view the steps of the usort() in PHP?

Question

This is the Example #1 from the php.net usort() page:

<?php
function cmp($a, $b) {
    if ($a == $b) {
        return 0;
    }
    return ($a < $b) ? -1 : 1;
}

$a = array(3, 2, 5, 6, 1);

usort($a, "cmp");

foreach ($a as $key => $value) {
    echo "$key: $value\n";
}
?>

The usort function takes the values within the array as pairs ($a-$b; so this is - 3-2, 2-5, 5-6, 6-1) and moves the $b value depending on whether the cmp() function returns -1, 0 or 1. If it is -1 the $b gets moved down (within a current pair), if it is 0 it stays in the same place and if it is 1 it gets moved up. This is how this is suppose to be working based on the top comment from the php.net manual usort() page.

Is there any way to see how this works step by step (the sorting process)? Am I able to see it or is it only possible to see the final result after the sorting is done? I want to understand fully how this process works.

Answer 1

Using some debug output in your comparison function you can only see the comparisons that PHP does, but can't see intermediate states of the array.

But the algorithm used in usort is well known - it's QuickSort ( Which sort algorithms does PHP's usort apply? ).

You can see its vizualisation at http://www.algomation.com/algorithm/quick-sort-visualization (or just google "quicksort algorithm visualization")