The Session object creates a new session id if the browser does not send a session cookie, so by the time your script runs the session id is already available.  Obviously this session id is not known by the browser until the session cookie is sent back to it with the rest of the script output, but this should not matter to you.

 

The only situation that may confuse things is that if a browser does not support cookies it will cause a new session id to be created with each request.  There are ways to get around this with some creative use of session variables, but you will need to wait until the second request from a particular user before recording a new visit.