session.gc_probability = 1
this directive states that there is a 1% probability that the garbage collection process is started on every session initialization
session.gc_maxlifetime = 7200
this directive states that after 7200 seconds stored data will be seen as garbage and can then be cleaned up by the garbage process
so after two hours have passed a session variable can be removed by the garbage collection process when a session is started with a probability of 1%
so you would expect the variable to last on the average for two hours plus 50 session initializations
paulsbooker