I feel that the feature I am going to describe is desirable for security reasons: 
The function session_start should return FALSE when the GET (or cookie) requests to resume a non-esistent phpsession. 

If there is already a (quick) way to prevent resuming non-esistent or closed phpsessions, please let me know, else please, add the feature to the TODO list.
(I've set session_cache_limiter('private') and I *don't* want to set it to 'nocache').
Many thanks
Siva