I don't know how it exactly use, but I just want to tell u how I did it.
A session has a lifetim, u can change this with: session_set_cookie_params(lifetime in minutes)
With session_get_cookie_params u get a array with ["lifetime"] back, when it's 0 the time is up and u can reset the variables.
I didn't use that one.
I save the begin time in a session variable and compare this with the current time, is the time up the session variables are to be reset.
unset("sessionvariable");
I can't think of another method.
Good luck :>
greezzz Anita