In PHP, if you sort -or better: do not sort- an array, using a custom function that happens to return 0 every time, the resulting order is undefined as opposed to the original order as you might expect.

So, if you do something like this and X is never set:

usort($arr_content['content']['results'], function($a, $b) {
    if (isset($a['X']) && isset($b['X'])) {
       if ($a['X'] == $b['X']) {
            return 0;
        }
        if ($a['X'] < $b['X']) {
            return -1;
        }
        return 1;
    }
    if (isset($a['X'])) {
        return -1;
    }
    if (isset($b['X'])) {
        return 1;
    }
    return 0; // this is returned for every element since $a['X'] and $b['X'] are never set
 });

you end up with an array that’s sorted in an unexpected way, in my case even perfectly reversed!

I had to work around this “feature” of PHP by checking the number of undefined X’es in the array and skipping the usort() of their number is equal to the number of elements in the array.