session_id() is not a variable in the session. It is the identifier for the session itself. You can't access it directly you would have to start a new session.
If you use:
That will completely remove the old session and all of it's contents when starting a new session. There is no such thing as ending a session and continuing to be on the website without a session_id() set. The only option is to generate a new one and start a new session so that it will have a new value.