Change this:
<input type="submit" value="Submit">
to:
<input name="submit" type="submit" value="Submit">
and change this:
if (($_SERVER["REQUEST_METHOD"] === "POST") && (isset($_SERVER["HTTP_REFERER"]) && strpos(urldecode($_SERVER["HTTP_REFERER"]), urldecode($_SERVER["SERVER_NAME"].$_SERVER["PHP_SELF"])) > 0) && isset($_POST)) {
to:
if (isset($_POST["submit"])) {