After your insert, the page redirects back to itself... that causes the session variable to be reset. I think it will work if you change the trigger on the initial session variable set code from:
<?php
@session_start();
if("" === "") {
$_SESSION["formStep"] = "1";
}
?>
to:
<?php
@session_start();
if(!isset($_SESSION["formStep"])) {
$_SESSION["formStep"] = "1";
}
?>