In a pointer from PHPKitchen.com today, there's a script highlighted for anyone out there looking to validate any kind of URL conforming to just about any kinds of rules.

Hi, I am doing an URL validator. It needs to be the best URL validator I ever made - and it is already quite functional.

Validating URLs with PHP

I'd be very happy if you'd go there and try to break it and tell me what went wrong if you do break it. In return you are free to copy the code for your own use. Usage is very simple, you construct an array with some key/value pairs - look in the source for which value pairs you can use.

He goes on to give a sample of the script, showing how simple it is to use. This class looks very powerful already, and it still has development to go...