Changing identities of user

Visitors to my website are 'authenticated' by starting a session when they login with their ID as a session variable and checking their identity against a database each time they visit a page carrying confidential information about themselves. I wish to allow *subsequent* visitors to use the same browser 'session' by having earlier users 'logout' and new users 'login' (this is in an environment where terminals are shared). Will some appropriate use of session_destroy() achieve this? I have tried but the user stubbornly insists on remaining the first logged-on user Some hints on how to control caching of confidential pages in these circumstances would also be welcome! I am using PHP4.0 and Apache 1.3.12 on Win 2k Quintin A Blane