For future reference, here is a "Redux" form file that shows how to handle a form on a single page.
>
$page_title = 'redux';
// Check if the form has been submitted.
if (isset($_POST['submitted'])) {
// handle form script here
} // End of main isset() IF.
// Leave the PHP section and create the HTML form.
?>
Build postal code file
</pre>
<form action="" method="post">
File:
</form>
Notice the 'hidden' submitted value that is used to determine whether to perform the php code. It ignores handling the form data and processing the php section unless the hidden value "isset", like on the initial presentation of the form.
As noted above, sometimes it is preferable to use the multiple files.