Is there something wrong with PHP session tracking?
Because PHP runs as ‘you’, not as the webserver, if you enable PHP session tracking, the temporary session files it creates are not writable by other users. As a result, if users visit two different user or society sites on the SRCF which both use PHP sessions, they’ll get an error message. The way to avoid this is for everybody to restrict their session cookies only to your own website (a good idea for security reasons in any case).