A problem about sessions

Hi all, I have a PHP script that takes very long time to execute. I manage it using the following structure: ob_start(); // some code session_start(); // some code header("Content-length: ".ob_get_length()); ob_end_flush(); // some other, very long, code! The problem I am facing is that, after having set up a session, often I have error as I don't find any simple variable that I wrote in $_SESSION. Correct my if I am wrong, but I think that $_SESSION get saved to disk only at the end of the script, after the "very long code" has been executed. There is a way to save the session manually just after ob_end_flush()? I can find a different solution, but I think it could be nice to have this working as is (if I am correct hunting this problem). Cheers, Tazio