使用Ajax和PHP更新购物车

Question

I am attemtping to use Ajax and PHP to create a shopping cart. At the moment I console.log and can see that the correct product ID is being selected when I click 'add cart' but nothing happens in my PHP when I try and var dump the POST data that should be sent. I need to be able to get that product ID in my PHP, search the database for the product and then add to the basket.

PHP

<?php
session_start();

include('classes/class-database-functions.php');

$database = new Database_Functions();

if($_POST) {

    var_dump($_POST);

}
?>

JS

$(document).ready(function(){   
        $(".form-item").submit(function(e){
            var form_data = $(this).serialize();
            var button_content = $(this).find('button[type=submit]');
            button_content.html('Adding...'); //Loading button text 

            $.ajax({ //make ajax request to cart_process.php
                url: "cart_process.php",
                type: "POST",
                dataType:"json", //expect json value from server
                data: form_data
            }).done(function(data){ //on Ajax success
                $("#cart-info").html(data.items); //total items in cart-info element
                button_content.html('Add to Cart'); //reset button text to original text
                alert("Item added to Cart!"); //alert user
                if($(".shopping-cart-box").css("display") == "block"){ //if cart box is still visible
                    $(".cart-box").trigger( "click" ); //trigger click to update the cart box.
                }
            })
            e.preventDefault();
        });
});