well, normally ppl would delete a cookie by setting the time to a time in the past instead of your "time() + ((5 6) 1)", and perhaps also setting the cookie's value to a null value.
as for "the best way to delete data from a mySQL DB based on that session_id when it expires", well, one way would be to check for the existence of that cookie, then act accordingly.
the problem, of course, is that the user doesnt have to access your script, so your script wouldnt really be able to act accordingly.
as such, you'll probably have to re-structure your script, or risk unreliability.
