值作为按钮中的变量未传递到页面

Question

I have created a website with cart for users. when the user clicks the add to cart button, they should be redirected to the cart page with the item displaying. The library.php which process the code for the cart is below:

<?php

// load database connection script
include("database_connection.php");

/*
 * Tutorial: PHP MySQL Shopping cart
 *
 * Page: Application library
 * */

class ShopingCart
{
    protected $db;

    function __construct()
    {
        $this->db = DB();
    }

    /**
     * get products list
     *
     * @return array
     */
    public function getProducts()
    {
        $query = "SELECT *  FROM `entertainment`";
        if (!$result = mysqli_query($this->db, $query)) {
            exit(mysqli_error($this->db));
        }
        $data = [];
        if (mysqli_num_rows($result) > 0) {
            while ($row = mysqli_fetch_assoc($result)) {
                $data[] = $row;
            }
        }

        return $data;
    }

    /**
        * get given product details
        *
        * @param [integer] $id
        * @return array
        */
       public function getProductDetails($id)
       {
           $id = mysqli_real_escape_string($this->db, $id);
           $query = "SELECT *  FROM `entertainment` WHERE `id` = '$id'";
           if (!$result = mysqli_query($this->db, $query)) {
               exit(mysqli_error($this->db));
           }
           $data = [];
           if (mysqli_num_rows($result) > 0) {
               while ($row = mysqli_fetch_assoc($result)) {
                   $data['id'] = $row['id'];
                   $data['title'] = $row['title'];
                   $data['price'] = $row['vendor_price'];
                   $data['quantity'] = 1;
               }
           }

           return $data;
       }

       /**
        * Add new product into the cart
        *
        * @param [integer] $id
        * @return void
        */
       public function addToCart($id)
       {

         $product = $this->getProductDetails($id);

         $isFound = false;
         $i = 0;

         if (!isset($_SESSION['shopping_cart']) || count($_SESSION['shopping_cart']) < 1)
         {
             $_SESSION['shopping_cart'] = array(0 => $product);
         } else {

             foreach ($_SESSION['shopping_cart'] as $item) {
                 $i++;
                 foreach ($item as $key => $value) {
                     if ($key == "id" && $value == $id) {
                         array_splice($_SESSION['shopping_cart'], $i - 1, 1, array([
                             'id' => $item['id'],
                             'title' => $item['title'],
                             'price' => $item['vendor_price'],
                             'quantity' => $item['quantity'] + 1,
                         ]));
                         $isFound = true;
                     }
                 }
             }
             if ($isFound == false) {
                 array_push($_SESSION['shopping_cart'], $product);
             }
         }
       }

       /**
        * remove existing product from the cart
        *
        * @param [integer] $id
        * @return void
        */
       public function removeProductFromCart($id)
       {
           unset($_SESSION['shopping_cart'][$id - 1]);
       }
}

?>

I am passing the value to this page using the button. the button is below

  <form method="post" action="cart.php" class="form-inline">
  <input type="hidden" value="' . $product['id'] . '" name="product_id">
  <input type="hidden" value="add_to_cart" name="add_to_cart">
  <button type="submit" name="id"  value="<?echo $getRowVenue['id'];?>" class="btn btn-primary">Add to Cart</button>
  </form>

when I am passing id like value="12"; then the product is added to cart. but on the index page, I am getting the id like $getRowVenue['id']; I have printed and checked but when I am passing this variable to the button, I am not getting the value, the item is not added to the cart.

</div>