calling session_start()

Hi All, When does session_start() have to be called? I have the following setting: A bunch of php pages are placed in a frameset. If someone tries to access one of these pages without the session-id cookie being set, the page calls a reloading of the frameset. The frameset page itself is the first to call session_start() Am I right to believe that session_start() has to be called before one can do if (!isset($_SESSION['foo']))? It is this test that determines whether or not to reload the frameset. On my development system this works just fine, but going into production on a web hotel, I get a lot more warnings. Every page in the frameset says "Warning: Cannot send session cache limiter - heaaders already sent..." in response to session_start() in the test I described above. Which setting in php.ini enables this warning? Do you know how I can test for the presense of a session without getting this error? Regards, Børge