Catching The 'Back' Button

I'm wondering if there's a way to catch when the user clicks the 'Back' button from a specific page.  When this happens, I need to run a system call that deletes everything in a session-specific directory.  The systems call would look something like this: system('rm -r /home/websites/genework/html/DATA/'.$Sess_ID.'/*');

So, the only thing left is the snag involving the catch of the 'Back' button.  Javascript has a way to catch this action (window.onbeforeunload), but the tap-dancing that would be required to get PHP to activate the system call is extremely ugly and I'm not even sure it'd work.