PHP, HTML form blank, session, and user going back in browser

Скраћена веза: http://pedja.supurovic.net/veza/8029

I stumbled on an issue that took some time to resolve.

I have created plain HTML form. After user submits form, he is lead to page that informs him if all fields content is valid or if there is any other problem with form.

At that point user may decide he wants to go back to form, change something and submit again. That worked quite fine. User clucks on Back button on his browser, it opens previous page, which is the form,it is prefilled with his data and everything was fine.

Until some time later I realized that it does not work that way any more. […]
[ ... види цео чланак ... ]