'password protecting' within existing php-sessions site

we have a website at a school i work at and restrict access to portions using php-sessions, and would like to extend the existing authentication scheme to the gallery - without having the users to have to re-login.

i've used the search function and have come up with a lot of people doing stuff, but not this- if this has been answered, or you know where I should look, please post a link

the site's current 'protected' .php pages have the following call at the top: <?php include('accesscontrol.php'); ?> and accesscontrol.php does the session checking/auth, etc. --- is the answer as simple as having that statement in the top of each gallery page (or a header file?) i'm not so concerned about the gallery 'admin' functions online, only a few people add pix, and they have XP on the desktop, so if they can't auth as gallery-admins onine from within their existing session that's fine.

thanks.