session variables exist for the length of the sesion (unless specified otherwise) which basically means for as long as they are in your site, so yes - session variables set in one php page will still be available even after visitng several html pages etc (in the same site) as long as the pages which need to access the session data all have the appropriate session_register("vars") set.
In fact if the user has cookies enabled they shoud even be able to go to a few other sites, come back and still be able to use the session data, but its best not to rely on that
MVC is the current buzz in web application architectures. It comes from event-driven desktop application design and doesn't fit into web application design very well. But luckily nobody really knows what MVC means, so we can call our presentation layer separation mechanism MVC and move on. (Rasmus Lerdorf)