I følge zend, er session.gc_maxlifetime den tid der skal gå fra sessionen sidst blev tilgået og til den skal slettes.
Zend skriver følgende:
(udpluk fra,
http://www.zend.com/zend/tut/session.php)
" * You can use the gc_maxlifetime configuration directive to determine how long after the last access to this session the data should be destroyed. This is used because the server doesn't know whether the cookie still exists on the client side. However, performing such a cleanup of old sessions (called "garbage collection") on every page request would cause considerable overhead. Therefore, in tangent with the gc_maxlifetime, you should use gc_probability. This specifies with what probability the garbage collection routine should be invoked. If gc_probability is 100, the cleanup is performed on every request (that is, with a probability of 100%); if it's 1 as by default, old sessions will be removed with a probability of 1% per request.
If you don't use cookies but pass the session ID via GET or POST instead, you need to pay special attention to the garbage collection routines. Users might bookmark URLs containing the session ID, so you need to make sure that sessions are cleared frequently. If the session data still exists when the user accesses the page with the session ID at a later time, PHP simply resumes the previous session instead of starting with a new session, which may not be your intention. A value of 10 to 20 for gc_probability would better fit this scenario than the default value of 1.
Tips
You might ask yourself why PHP allows you to specificy a probability (gc_probability) which determines when garbage collection will occur, rather than a function which cleans up every n times. If PHP used a counting function, the server would need to track the number of opened sessions somehow. Using the probability function means that the server does not have to store counters for cleanup, which translates to cleaner and faster execution. "