HTTP expire

I want to cashe database driven forum, but SESSION is used for passing
userID. When I use code below offline browsing is ok, but session variable
is difrent on cashed pages. Some pages stored gost id some admin. How can i
solve this.

<?php
ob_start();
session_cache_limiter('private');

header("Cache-control: private");
header("Content-Type: text/plain");
header("Pragma: public");

session_start();
?>
<!-- SADR®AJ -->
<?php
ob_end_flush();
?>