Your comparison function is inconsistent: for example, it will claim that l=a (neither value depends on the other) and that t=l (same) yet it would claim that a<t. PHP 5.6 does not attempt that comparison because (a) that was sheer luck based on the ordering of the original array, but more importantly (b) it could/should logically infer that a=t based on the previous results.

PHP 7's sorting behavior changed and the problem is no longer apparent for that given ordering; I tried shuffling randomly a couple times but didn't get any incorrect results, however it's quite possible there is at least one original array ordering that will expose the same flaw.

PHP's quicksort is not really the right tool for this job. A topological sort is the most appropriate answer in theory but a simple insertion sort may be sufficient.

Regardless, 5.6 has reached end of life and is only receiving security-related fixes now.