Probably a couple ways of doing this. The one that comes to my mind is to use [man]curl[/man]. You can setup curl to just bring you the header (which is all you want - you don't want this script to actually tie up bandwidth downloading the PPT file).
Actually, now that I think about it, you may not even have to go this far. All you're really interested in is check the response code returned which will be the same code you would get if the browser could find the file (so say a 404 error, you'd have the code look for a 404 error - although making it more generic will make your script able to detect if the server is down or something like that).
Either way, I'm not sure how big the PPT files are, but I'd try not to download them because it will tie up how much time it will take this script to find out if the file exists or not. A simple check of the headers will tell you if its a 404 or of the file was found.
The other method might be to use fsocket functions or even just try file() or [man]file_exists[/man] (in fact, just checking file_exists, you'll find some snippets of code which appear to do what you're looking to do).
