Thanks Paul and Tom, my problem is solved. It had indeed something to do with the order of the index files, as stated in the Apache properties. Besides the index.php there appeared to be an index.html file in the same directory (which I never noticed before), which was addressed first, before index.php, that way causing my problem. Why it díd work properly before I did the PHP server upgrade is probably because the order of the index files was set to php above html before the upgrade, but of course I can't confirm that anymore.
Anyways, this problem has taken me too much time, now let's continue doing fun stuff.