数量增加或减少时，总计应该放什么？请帮助我

Question

1. 数量增加或减少时，总计应该放什么？我想发生当用户点击数量增加时总数也增加。我尝试了一些 javascript，但它似乎不起作用。不知道放什么

2. 。然后这是我在 order.php 中的代码块：

            <?php
            include("functions.php");
            include("footer.php");
   
            global $lastname,$firstname,$address,$email,$contactno,$username,$customer_id;
   
            $id = initObj('id');
            $custom_text = initObj('custom_text');
   
            if ($sess_username!="") {
                $sql = "select * from customers where username='$sess_username'";
                $row = getRow($sql);
                $customer_id = $row['id'];
                $lastname = $row['lastname'];
                $firstname = $row['firstname'];
                $address = $row['address'];
                $email = $row['email'];
                $contactno = $row['contactno'];
            }
   
            fHeader();
            ?>
            <div class="container mt-4">
                <h3>Order Form</h3>
                <div class="row mt-4">
                    <div class="col-md-10">
                        <!--products-->
                        <div class="row">
                            <?php
                            $sql = "select * from products where id=$id";
                            $row = getRow($sql);
                            $prodcode = $row['prodcode'];
                            $prodname = $row['prodname'];
                            $proddesc = $row['proddesc'];
                            $prodimage = chkPic("product",$prodcode);
                            $total = $row['price'];
                            $custom_image = chkPic("order",genTransNo("N",$uid));
                            ?>
                            <div class="col-md-5">
                                <div class="card" align="center" style="overflow: hidden;">
                                    <center>
                                    <!-- <img src="<?=$prodimage?>" class="card-img-top" alt="<?=$prodname?>"> -->
                                    <div style="width:400px; height:468px; margin-left: -10px; background-image: url(<?=$prodimage?>?v=<?=time()?>); background-size:400px 468px;">
                                        <center>
                                            <img src="<?=$custom_image?>?v=<?=time()?>" style="margin-top:150px;" width="40%" height="40%"><br>
                                            <label style="margin-top:10px; font-size:20px; font-weight:bold;"><?=$custom_text?></label>
                                        </center>
                                    </div>
                                    <div class="card-body">
                                        <h5 class="card-title"><?=$prodname?></h5>
                                        <p class="card-text"><?=$proddesc?></p>
                                    </div>
                                    </center>
                                </div>
                            </div>
                            <div class="col-md-7">
                                <form name="frm" method="post" action="confirm.php" role="form" enctype="multipart/form-data">
                                    <div class="box-body">
                                        <table width="100%" border="0" cellspacing="2" cellpadding="2">
                                            <tr>
                                                <td colspan="2"><label class="h3">Customer Information</label>
                                                <?php if ($sess_username=="") { ?>
                                                <span style="margin-top:10px; float:right">Already have an account? <a href="login.php?id=<?=$id?>">Login</a></span>
                                                <?php } ?>
                                            </td>
                                            </tr>
                                            <tr>
                                                <td width="20%"><b>Last Name</b></td>
                                                <td width="80%">
                                                    <div class="col-md-12"><input name="lastname" type="text" class="form-control" value="<?=$lastname?>" required></div>
                                                </td>
                                            </tr>
                                            <tr>
                                                <td width="20%"><b>First Name</b></td>
                                                <td width="80%">
                                                    <div class="col-md-12"><input name="firstname" type="text" class="form-control" value="<?=$firstname?>" required></div>
                                                </td>
                                            </tr>
                                            <tr>
                                                <td width="20%"><b>Address</b></td>
                                                <td width="80%">
                                                    <div class="col-md-12"><input name="address" type="text" class="form-control" value="<?=$address?>" required></div>
                                                </td>
                                            </tr>
                                            <tr>
                                                <td width="20%"><b>Email</b></td>
                                                <td width="80%">
                                                    <div class="col-md-12"><input name="email" type="text" class="form-control" value="<?=$email?>" required></div>
                                                </td>
                                            </tr>
                                            <tr>
                                                <td width="20%"><b>Contact No.</b></td>
                                                <td width="80%">
                                                    <div class="col-md-12"><input name="contactno" type="text" class="form-control" value="<?=$contactno?>" required></div>
                                                </td>
                                            </tr>
                                            <tr>
                                                <td width="20%"><b>Remarks</b></td>
                                                <td width="80%">
                                                    <div class="col-md-12"><input name="remarks" type="text" class="form-control" value=""></div>
                                                </td>
                                            </tr>
                                            <tr>
                                                <td width="20%"><b>Custom Text</b></td>
                                                <td width="80%">
                                                    <div class=""><input name="custom_text" type="text" class="form-control" value="" required></div>
                                                </td>
                                            </tr>
                                            <tr>
                                                <td width="20%"><b>Custom Text Back</b></td>
                                                <td width="80%">
                                                    <div class=""><input name="custom_text_back" type="text" class="form-control" value="" required></div>
                                                </td>
                                            </tr>
                                            <tr>
                                                <td width="20%"><b>Custom Text Sleeves</b></td>
                                                <td width="80%">
                                                    <div class=""><input name="custom_text_sleeves" type="text" class="form-control" value="" required></div>
                                                </td>
                                            </tr>
                                            <tr height="40">
                                                <td width="20%"><b>Custom Image</b></td>
                                                <td width="80%">
                                                    <input name="file" type="file" required><br>
                                                    <!-- <img id="custom_image" src="<?=$custom_image?>?v=<?=time()?>" class="my-2" width="30%" height="30%"> -->
                                                </td>
                                            </tr>
                                            <tr height="40">
                                                <td width="20%"><b>Custom Image Back</b></td>
                                                <td width="80%">
                                                    <input name="back" type="file" required><br>
                                                    <!-- <img id="custom_image" src="<?=$custom_image_back?>?v=<?=time()?>" class="my-2" width="30%" height="30%"> -->
                                                </td>
                                            </tr>
                                            <tr height="40">
                                                <td width="20%"><b>Custom Image Sleeves</b></td>
                                                <td width="80%">
                                                    <input name="sleeves" type="file" required><br>
                                                    <!-- <img id="custom_image" src="<?=$custom_image_sleeves?>?v=<?=time()?>" class="my-2" width="30%" height="30%"> -->
                                                </td>
                                            </tr>
                                            <tr>
                                            <td width="15%"><b>Quantity</b></td>
                                            <td width="85%">
                                                <div class="col-md-2"><input name="qty" type="number" class="form-control" value="" required></div>
                                            </td>
                                            </tr>
                                            <tr>
                                                <td width="20%"><b>Total</b></td>
                                                <td width="80%">
                                                    <div class=""><input name="total" type="text" class="form-control" value="<?=$total?>" readonly></div>
                                                </td>
                                            </tr>
                                        </table>
                                    </div>
                                    <div class="box-footer mt-3">
                                        <input name="product_id" type="hidden" value="<?=$id?>">
                                        <input name="item" type="hidden" value="<?=$prodname?>">
                                        <input name="customer_id" type="hidden" value="<?=$customer_id?>">
                                        <button name="action" value="save" type="submit" class="btn bgclrCyan">Confirm Order</button>
                                    </div>
                                </form>
                            </div>
                        </div>
                    </div>
                </div>
            </div>
            <?php Pfooter(); ?>
            <script src="js/lightbox-plus-jquery.min.js"></script>
                <script>
                $("#headerLogo").click(function(){
                    window.location.href="index.php";
                })
            </script>