session ID creation

when a forum system prints a session ID into the db, where do they get the session from to look like 'd5dc0ea2175aa1962d6ad24fa2cc4a89' ? i looked at the IPB code and it just says $poss_session_id = "";

session_id - stores the session ID (oddly enough)

http://php.us.themoes.org/manual/en/function.session-id.php

As for that piece of code you have there, if im not mistaken its resetting the variable $poss_session_id to nothing.