Last month there were at least two inquiries at the Bangalore PHP UG on how to get PHP to communicate with the server's serial port. I didn't think it was possible, but here's what a quick google search turned up...

Two tools popped up on the radar - the PHP Serial Extension and the PHP Serial Class. Both have their benefits, and both can be used for the same kinds of functionality - one for the Windows side and the other for Linux-ish machines. The first seems to be a bit more flexible in its approach, though.