string session_id(string [id]);
session_id() returns the session id for the current session. If id is specified, it will replace the current session id.
directory used to save session data. If path is specified, the path to which data is saved will be changed.
The constant SID can also be used to retrieve the current name and session id as a string suitable for adding to URLs.
Note: This function was added in PHP 4.0.