session lost when back button hit?

I have two pages, page one links to page two via a form button. The browser does not accept cookies. On page two I create a session variable with: $_SESSION["id"] = 1; But if I hit the back button to go back to page one $_SESSION["id"] is not set. If I hit the reload button on page one it is still not set. How can I get a session var to stay set when someone hits the back button? Jc PS I have session.auto_start ON and enabled trans-sid