note 8427 added to session_encode

Yes, that is correct. As sites get larger, it becomes harder to host a site off of one machine. Many sites use commercial grade load-balancers to transparently switch users between machines. In order for sessions to really be persistant, they have to be accessible by all servers in the 'farm'. Saving them to a file on the local system (default behaviour) does not allow for this. Using a database to store a restorable version of the session is a widely used solution, and these functions allow for just that. http://www.php.net/manual/function.session-encode.php