We can create a unique number for the session of the browser by generating a session id. Session id is generated by PHP at server end user can't change the generated session id. However it can be re-created.
Each time we login by our userid and password we get a new session_id. A particular session_id may be associated with a userid but it keep on changing each time the same member login.
If there are ten members logged in to your website at a moment then there will be ten different session id and each member will be associated with one session id.
Re generating the session_id
By using script we can regenerate another session_id. Here is the function .
Note that this is not same as session destroy or logout, here we are only changing the session_id value to a new one. All other session data remains unchanged.
PHP Session variable creating checking and destroying using session_start() using userid and name