two questions about usning session.

Hello, I have two questions about using sessions in PHP. Firstly, I use session to remeber the username and a session ID for the given user. When I hit the BACK button on the browser (IE), it always set that my page is expired and ask me to click the refresh button. How to prevent this from happening? Secondly, how is the PHP session ID be computed? When user A login, a session is started and his name nad session ID are stored. Now, on the same PC, I start another instance of the IE browser, login as User B, her name and session ID are stored under the same PHP session file. I switch back to the Windows of User A and continue to do something, the PHP session gives me the name of User B and User'B session ID. Why? Am I doing something wrong here? Thanks, - JOE