session only for who needs it

I have a site in wich user can register, but everyone can navigate as Guest, even if he/she is nt registered. The problem is that I need session for registered user, but I don't need them for unregistered ones! So I tried to check if they already have an opened session (they are registered), but to check it I have to do a session_start(), and this will OPEN a session if it's not opened! So I'll have an opened session for every guest I have in the site. This is difficult for me because I would like session to last very long, and many empty session waste a lot of space! so session_start() must be like void session_start() { if( [cookie_on_client] == [sess_file_on_server] ) { look_up_the_file [...] } else { // create new session create sess_file_on_server give corresponding cookie to client } ... } but I would need a function like bool session_check() { if( [cookie_on_client] == [sess_file_on_server] ) { return true; } else { return false; } } Is it possible to do it with the PHP session API, without hacking the code, go and read the session files,...? any hint is appreciated. bye Mattia Cazzola