page expired message in php script

i am working on an e-commerce website using php. i 开发者_如何学编程have tried post/get and now session but problem is persisting. when i hit back button it display message page has expired. i thought may be i am not handlling session variables proparly but i dont know what to do ? what could be cause ans what could be solution .

You're probably seeing the form submission "page is expired" page. If you're submitting forms and you attempt to back up, that page is expired, you have to resubmit the form data to view the page again. Just refresh the page.

See http://support.microsoft.com/kb/183763