Your PHP setup is probably sending headers along with the reply, telling the browser to not cache the page. Therefore, when going to the previous page, it will be reloaded as if it were a new page. You can tweak the PHP settings to not have PHP sending these headers. Edit the php.ini and set the "session.cache_limiter" directive to have no value at all. So the line in php.ini would end up like this:

        session.cache_limiter =
      

After editing the php.ini, restart the webserver. From then on, using the back button should work like expected.

If you do not have access to php.ini (you are using a hosting account), submit a request to your host asking that they change the aforementioned parameter for you. Most hosts should be willing to do this in the majority of cases.