我正在用php和mysql做我的电子商务网站的学期专题。我用这两种工具构建了一个购物车系统。我遇到的问题是，当购物车只包含一个项目时，它将按预期工作，即更新数据库“数量”列中的反映数量。但是当我增加购物车中的商品时，它将不会像我想的那样工作，即它只会用最后一个商品数量更新整个数据库的数量列。
下面是我的代码。
任何帮助都会很有帮助，因为现在我正在学习这些东西。

**欢迎客人！**购物车-商品：-价格：-转到购物车

<div id="products_box"><!-- gets the products from the database section starts from here -->
                <form action="cart.php" method="post" enctype="multipart/form-data">
                    <table width="750px" align="center" bgcolor="#0099CC">
                        <tr align="center">
                            <td><b>Remove</b></td>
                            <td><b>Product(s)</b></td>
                            <td><b>Quantity</b></td>
                            <td><b>Total Price</b></td>
                        </tr>
                        <?php
                        $ip_add = getIp();
                        $total = 0;

                        $sel_price = "select * from cart where ip_add = '$ip_add'";
                        $run_price = mysqli_query($db, $sel_price);

                        while($record = mysqli_fetch_array($run_price)){
                            $product_id = $record['product_id'];

                            $pro_price = "select * from products where product_id = '$product_id'";
                            $run_pro_price = mysqli_query($con, $pro_price);

                            while($p_price = mysqli_fetch_array($run_pro_price)){

                                $product_price = array($p_price['product_price']);
                                $product_title = $p_price['product_title'];
                                $product_img = $p_price['product_img2'];
                                $single_price = $p_price['product_price'];

                                $value = array_sum($product_price);
                                $total += $value;

                        ?>
                        <tr>
                            <td><input type="checkbox" name="remove[]" value="<?php echo $product_id;?>"></td>
                            <td><?php echo $product_title;?><br><img src="admin_area/products_images/<?php echo $product_img;?>" width="80px" height="80px"></td>
                            <td><input type="text" name="qty" value="1" size="3" min="1"></td>
                            <?php 
                                if(isset($_POST['update'])){
                                    $qty = $_POST['qty'];

                                    $insert_qty = "update cart set qty='$qty' where ip_add='$ip_add' AND product_id = '$product_id'";

                                    $run_qty = mysqli_query($con, $insert_qty);

                                    $total *= $qty;
                                }
                            ?>
                            <td><?php echo  $single_price ." ". "&euro;"?></td>
                        </tr>
                        <?php
                            } // to include in the php code so it effects the whole row
                        }?> 
                        <tr>
                            <td colspan="3" align="right"><b>Sub Total:</b></td>
                            <td><b><?php echo $total ." ". "&euro;"?></b></td>
                        </tr>

                        <tr></tr>
                        <tr></tr>
                        <tr></tr>

                        <tr>
                            <td colspan="2"><input type="submit" name="update" value="Update Cart"></td>

                            <td> <input type="submit" name="continue" value="Continue Shopping"></td>
                            <td><button><a href="checkout.php" style="text-decoration:none; color:black;">Checkout</a></button></td>
                        </tr>
                    </table>
                </form>