AJAX and PHP code not working - php

Question

I am sending my form data through AJAX but for some reason my PHP script is not running. The test echo's Im using in my PHP script is not showing. The window.alert("success") does show but HIDE and SHOW form1 and form2 also does not work.
Here is the code:
$('#mainform').on('submit', function(event) {
//test for empty fields
//test for Bots
//insert data into DB
//pass t_code on to next form
//create a page number for tabs
event.preventDefault(); //stops form on submit
var a = document.forms["mainform"]["hidden"].value;
if (a === ""){
var formData = {};
$.each($("#mainform").serializeArray(), function (i, field) {
formData[field.name] = field.value;
});
$.ajax({
url: 'insert_tut_description.php',
data: formData,
method:'POST',
success: function(response) {
window.alert("success");
pnum = 1;
t_code = form.elements["t_code"].value;
$("#form1").hide();
$("#form2").show();
document.getElementById("pnum").innerHTML = pnum;
}
});
};
});
<?php
echo "php running";
require 'config/config.php';
$t_title = $conn->real_escape_string($_POST['t_title']);
$t_code = $conn->real_escape_string($_POST['t_code']);
$t_image = $conn->real_escape_string($_POST['t_image']);
$hidden = $conn->real_escape_string($_POST['hidden']);
$t_desc = $conn->real_escape_string($_POST['t_desc']);
$t_url = "something.php";
echo $t_title;
echo t_url;
if(empty($hidden)){
echo "hidden is empty";
$query = "INSERT into tutorial_list (title, description, t_code, t_url, image,) VALUES('" . $t_title . "','" . $t_desc . "','" . $t_code . "','" . $t_url . "','" . $t_image . "')";
$success = $conn->query($query);
if (!$success) {
die("Couldn't enter data: ".$conn->error);
} else {
$conn->close();
}
}
?>