你好，你似乎还没有添加任何东西来通过post方法。找到下面的代码，我已经告诉你如何做正确的方式。这将添加所选的产品到该特定的变量每一次。

while ($row_all = mysqli_fetch_assoc($result_all)) 
                {
                    echo '<form method="post">';
                        echo '<u>'.$row_all["name"].'</u>';

                        echo '<br>';

                        echo '<button name="add_to_cart" value='.$row_all['name'].' type="submit">Add to Cart</button>';

                        echo '<hr>';
                    echo '</form>';

                }

            if(isset($_POST["add_to_cart"]))
            {

                //CREATE A VARIABLE THAT HOLDS THE SELECTED PRODUCTED TO BE ADDED TO CART
                    $selectedProduct = $_POST["add_to_cart"];

                echo 'Selected Product  = '.$selectedProduct;

            }

    mysqli_close($conn);

但是，如果你希望它每次添加或迭代产品的数量，以便变量存储许多产品，你将不得不使用会话变量

<?php 

  include('connect.php'); 
  require("session.php"); 

        //create the sql statement 
        $sql_all=
             "SELECT * FROM product 
              ORDER BY product_ID DESC";
        $result_all = mysqli_query($con,$sql_all);
while ($row_all = mysqli_fetch_assoc($result_all)) 
                {
                    echo '<form method="post">';
                        echo '<u>'.$row_all["name"].'</u>';

                        echo '<br>';

                        echo '<button name="add_to_cart" value='.$row_all['name'].' type="submit">Add to Cart</button>';

                        echo '<hr>';
                    echo '</form>';

                }

            if(isset($_POST["add_to_cart"]))
            {

                //CREATE A VARIABLE THAT HOLDS THE SELECTED PRODUCTED TO BE ADDED TO CART
                    $selectedProduct = $_POST["add_to_cart"];

                echo 'Selected Product  = '.$selectedProduct;

            }

        mysqli_close($conn);
?>