A simpler way to achieve what you want is to use redirect on submit. After you process a POST request you redirect, possibly even to the same page. This is a common pattern called "Redirect after POST" or POST/Redirect/GET.
For example:
<?php
if($_POST) {
// do something
// now redirect
header("Location: " . $_SERVER["REQUEST_URI"]);
exit;
}
?>
<html> ...
<form method="post" action=""> ... </form>
By setting the action to "" then it will submit to itself, at which point the if($_POST) code block will validate to true and process the form, then redirect back to itself.
Of course you probably want to redirect to a different page that shows a "your form has been submitted" response or put the form on a different page and have the HTML of this page be the response.
The benefit of this method is that when you hit the back button it does a GET request so the form is not re-submitted.
On Firefox, it will actually take the submission to itself out of the browser history so when users browse across the web and then hit back, instead of seeing the "thank you" page they see the form page.
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