Which unique field to track during a PHP4 Session?

Hi- I am looking for some security advice with respect to session tracking using the PHP4 built-in session support. Both the Userid and the Username are unique keys in my database, but only the userid is seen in other related tables. With this in mind, when tracking user sessions from page-to-page throughout my site, what variable is "safest" to pass from page to page, so information can be extracted from the database when required? 1)userid 2)username (which is public, seen in message board posts) 3) one of the above AND the user's password. Thanks for any hints in this area! -jason