I coded a page in php.
But when hitting the browzers back button you get the following:

"Data Missing

This document resulted from a POST operation and has expired from the cache. If you wish you can repost the form data to recreate the document by pressing the
reload button. "

This is completely undesireable.
How can i fix this?
I know what this means but i don't know why exactely i get it.
The page i'm trying to do looks something like this:

if($action=="index") {
do some stuff
if($action=="billing") {
do some other stuff

then the action value gets cahnged and the page reloaded (throug links using $PHP_SELF)

Pleas help.