my setp is i have index.php (base file) calling all my other html / php scripts.
at the top of all my php scripts i have
if(unset($_SESSION['username'])die'You need to be logged in to see this page';
Which forces the page to stop loading b/c the user is not logged in.
I guess i do not understand your setup. IF that other script uses session that script should work. If you wish.. just make the entire modfication in an if statment that way it will not mess up the other script.
other then that.. I'm out of idea's for i do not understand the other script that you have calling this one.