May be I missed something, but
1/ session_start() will flock() the session file
2/ there is no way to unlock the file, except
   session_destroy(), but session_destroy() will
   unlink the session file (therefore session
   cannot be reused).
My need is : user opens 2 web pages (both multipart/mixed,
i.e. the scripts never end), 
none of them are changing the session variables, 
but both of them need their value.
I therefore need a session_end() that does exactly 
the same job as session_destroy() except it does
not call PS(mod)->destroy.
 
(changes are in session.c and php_session.h)