time    Returns the number of non-leap seconds since whatever time the
               system considers to be the epoch (that's 00:00:00, January 1,
               1904 for Mac OS, and 00:00:00 UTC, January 1, 1970 for most
               other systems).  Suitable for feeding to "gmtime" and "local-
               time".

##</code><code>##

if ( (time() - $timestamp) > (60 * 60 * 2) ) {
    # Do what you need to expire the session
}