Get data from URL PHP

Get data from URL PHP

I want to have a system that looks for a page at that URL and if it cant
find one it passes the URL to the last page it can find (sorry for the bad
explanation, this is the only way I could think of putting it). Example:
say I entered the URL "www.mysite.com/users/username" it would pass the
string "username" to "users.php" instead of having it like
www.mysite.com/users?user=username. if there is no users.php in the site
then it directs you to a 404 page.
Sorry for the really bad explanation all I really need is a name of what
this would be called and I could find some information on it but I am not
sure what I am looking for.
Thanks.