Likey . . .

I've got good news, and I've got bad news:
The universe is merely a figment of my imagination.
Now are you ready for the bad news?

Internet Explorer returns the highest resolution monitor on multi-monitor setups. This can cause problems. At least it does on mine. Firefox has no such problem.
Also remember that doing things like centering based on screen resolution is a bad idea because the browser window may not be maximised.

I forgot to add that you should take the whole snippet and save it as get_resolution.php. Then follow the instructions in the comments at the top.