Reformatting the URI on return to the browser

PHP, I've implemented "friendly urls" in my PHP scripts, so that people can type in things like: http://website.com/Cats And Dogs ... and they'll be taken to the right page. However, when they get to the page, the url will read: http://website.com/Cats%20And%20Dogs Not so pretty. I've noticed that Wikipedia will alter the URI to become more readable, by replacing the spaces with underscores, like so: http://wikipedia.org/wiki/Cats_And_Dogs I'd like to be able to do that with my system, but I can't quite figure out where in the process of getting a URI request and returning a page that the URI will get set. Can anyone help me with figuring out at what point I can gain control over the URI sent back, as Wikipedia does? Thanks for any advice. -- Dave M G Ubuntu Feisty 7.04 Kernel 2.6.20-15-386