Some questions.

Hello I'm a Chinese university student,I want to ask some questions about session. These days I'm build a website for my university with PHP, But I meet a question when I develop the part of User Management: After I have log out from a user page(I use "session_unset()" and "session_destroy()"),I can return to the page again by click the button "Back"to that pagea and refresh it, the user page can be shown again. This is unsafe. So I want to ask that the function "session_unset" and "session_destroy()" will destroy session immediately or there is a life-time for session. In my memory, I think that there is a life-time for session and the life-time can be configured. Another question: If the user log page is "main.php",the page for authenticate the user is "login.php" I use session to store the infomation of user such as : session_register($userid); But if the variables in the session are unfortunately be known by somebody else. and he can visit others' information bye the url:"login.php?userid=***",how can solve these problem? use a ugly but difficult session varable? Just two questions. Thanks a lot. Wish back soon! Best wishes