php not adding data to database by using file.php?data=value - php

Question

I have a form that I want to submit it without reloading the page with jquery. I am trying to make a form that will send the data to MySQL database with PHP.
This is the code in submit button onclick:
e.preventDefault();
var data = JSON.stringify($("form").serialize());
$.post("insert.php?" + data);
and this is the PHP file :
<?php
$username = $_POST['username'];
$password = $_POST['password'];
$gender = $_POST['gender'];
$email = $_POST['email'];
$phoneCode = $_POST['phoneCode'];
$phone = $_POST['phone'];
if (!empty($username) || !empty($password) || !empty($gender) || !empty($email) || !empty($phoneCode) || !empty($phone)) {
$host = "localhost";
$dbUsername = "use";
$dbPassword = "user_pass";
$dbname = "test";
//create connection
$conn = new mysqli($host, $dbUsername, $dbPassword, $dbname);
if (mysqli_connect_error()) {
die('Connect Error('. mysqli_connect_errno().')'. mysqli_connect_error());
} else {
$SELECT = "SELECT email From register Where email = ? Limit 1";
$INSERT = "INSERT Into register (username, password, gender, email, phoneCode, phone) values(?, ?, ?, ?, ?, ?)";
//Prepare statement
$stmt = $conn->prepare($SELECT);
$stmt->bind_param("s", $email);
$stmt->execute();
$stmt->bind_result($email);
$stmt->store_result();
$rnum = $stmt->num_rows;
if ($rnum==0) {
$stmt->close();
$stmt = $conn->prepare($INSERT);
$stmt->bind_param("ssssii", $username, $password, $gender, $email, $phoneCode, $phone);
$stmt->execute();
echo "New record inserted sucessfully";
} else {
echo "Someone already register using this email";
}
$stmt->close();
$conn->close();
}
} else {
echo "All field are required";
die();
}
?>
But it's not adding data to the database. I tried also writing the URL then the data like this: insert.php?data=value.But it didn't work. I am new to server-side and PHP, so I appreciate any help