omoutop - 1:00 pm on Sep 20, 2013 (gmt 0) [edited by: omoutop at 1:05 pm (utc) on Sep 20, 2013]
Do you by any chance use php 5.3?
There is a similar problem mentioned in official php site here [php.net].
The problem is that you use session_id() after session_start() (if i understand the situation correctly)
Edit: if you put your session_start() at the very top of your page, your ssession_id() maintains its value (tested in 5.1.4 and 5.3.3)
[edited by: omoutop at 1:05 pm (utc) on Sep 20, 2013]