Reauthenticate

Hi All, I'm in trouble to authenticate each user (using a database). When the user sucesfully login on your area the php script send a cookie, and every php script reads this cookie. So when the user log ou the php script expires the cookie and then the user cannot bookmark the page and turn back...... but, if the user clicks the 'logout' button, I expire the cookie e show the main screen, but, by other side, if the user click at 'back button' then he can see the page....... I would like to: - Each page look for the cookie, even if the back button is pressed. - If the cookie isn't there, then a 'expires page' is showed.... Which is the better way to do it ??? Thanks in advance, Marcelo Pereira Programmer