[eluser]WanWizard[/eluser]
The session_id is an internal key, and should not be used for this purpose. Regeneration is key in preventing session hijacking, and should NOT be disabled.
If you need this functionality, either store the ID you want to check in the userdata in such a way that you can query it reliably via a regex, or add an extra field to the session table that you can query.
I use the same technique to list who is currently logged in, and when they last had an interaction with the application (for which I also added a last_update field).