I think what you're looking for is mod rewrite. Can be done using .htaccess files. Do a search on google and you'll find plenty on it. I think it may even have it's own forum somewhere.
One suggestion: if you're making SE friendly urls, drop the ? all together.
For example:
http://www.yourdomain.com/products.php?id=45
to
http://www.yourdomain.com/tennis-rackets/wilson/nPro-serve
If you just change to
http://www.yourdomain.com/products.php?nPro-serve
your url still appears dynamic to search engines because of the ?, which defeats the whole purpose of SE friendly urls. You don't have to go as far as my example, but for sure drop the ?