Is it possible to change a PHP session variable from within Javascript?
Change a PHP Session in Javascript
you could probably do it by setting up a script that does the work of changing the session var, and make an ajax call to that script in your JS code. you'd definitely want to take steps to secure the php script if you do it this way
Usually sessions are carried from page to page with a cookie called "PHPSESSID". I assume that if you reset it or change it you are able to switch between sessions.