Well, the problem is that SCRIPT_FILENAME just contains the path to the script, it doesn't contain anything after it.

So, something like this:

https://www.ubbcentral.com/forums/ubbthreads.php/ubb/showflat/Number/216111

SCRIPT_FILENAME only contains:

https://www.ubbcentral.com/forums/ubbthreads.php/ubb/showflat

It chops off the actual data that we need to know what topic to show.