如何使用AJAX将数据发布到php文件[重复]

I am having trouble sending data to a php file to be processed. I have tried just about everything but cannot find the source of the problem. Below is a php file that sends a product name, price, and ID to a checkout function after a user clicks on a buy button.

<?php

      $servername = "localhost";
      $username = "root";
      $password = "root";
      $dbname = "Test";

        // Create connection
      $conn = new mysqli($servername, $username, $password, $dbname);

        // Check connection
      if ($conn->connect_error) {
       die("Connection failed: " . $conn->connect_error);
     } 

     $sql = "SELECT P1.Product_Name, S2.Price, P1.Product_ID FROM Product P1, Sale_Item S2 WHERE P1.Product_ID=S2.Product_ID AND P1.Category='Sports'";
     $res = $conn->query($sql);
     $counter=0;

     while ($row = $res->fetch_assoc()){

      $Product_Name = $row["Product_Name"];
      $Price = $row["Price"];
      $Product_ID = $row["Product_ID"];

      echo ('<td><p></p>'.$row["Product_Name"].'<br>'.$row["Price"].'<p></p><input type="button" value="Buy" onclick="checkout(\'' . $Product_Name . '\', \'' . $Price . '\', \'' . $Product_ID . '\')"</td>');          
      $counter++;

      if ($counter==3) {
        $counter=0;
        print "<br>";
      }
    }

    $conn->close();
    ?>

And next the checkoutfunction:

<script type="text/javascript">
        function checkout(Product_Name, Price, Product_ID) {
          //document.write(Product_Name, Price, Product_ID)
          var theProduct_Name = Product_Name;
          var thePrice = Price;
          var theProduct_ID = Product_ID;

          $.ajax({
            type: "POST",
            url: "http://localhost:8888/checkout.php",
            data: {Product_Name: theProduct_Name, Price: thePrice, Product_ID: theProduct_ID},
          });

          window.location.assign("http://localhost:8888/checkout.php")
        }
      </script>

I am using MAMP's phpMyAdmin's database. Is my url incorrect? I've tried using "http://localhost:8888/checkout.php" and just checkout.php. Below is the php file where I need to process data. For simply learning how to send the data I am just echoing inside the file to make sure it is actually posting. But nothing is being echoed.

<?php 

  session_start();
  $servername = "localhost";
  $username = "root";
  $password = "root";
  $dbname = "Test";

  // Create connection
  $conn = new mysqli($servername, $username, $password, $dbname);

  // Check connection
  if ($conn->connect_error) {
   die("Connection failed: " . $conn->connect_error);
 } 

  $theProduct_Name = $_POST['Product_Name'];
  $theProduct_ID = $_POST['Product_ID'];
  $thePrice = $_POST['Price'];

 echo $theProduct_Name.$theProduct_ID.$thePrice;

 ?> 

I am new to web-programming so any help or tips would be appreciated. I've been looking at this for hours now and cannot seem to get it to work.

</div>