gettin parameters from url, architecture question

Hi there, I recently dumped into a site which was kind of search engine optimized. They had urls like this: server.com/country_province_city_222.html Now I am wondering how they did this. I am aware on how to do it if you could at least have this: server.com/country/province_city_222.html where country would be a php file, but how are those guys picking the right file? Where is the php file stored and how does the server know which file to call?? Can anybody shed a bit of lite on that? ;-) Thanx, merlin