为什么我的提交按钮没有重定向

Question

I have a form with a submit button.

<input type="submit" name="submitOrder" id="submitOrder" value="Purchase" class="btn btn-warning">

Below this I have a couple lines of code:

<?php
    if (isset($_POST['submitOrder'])) {
        header ("location:payment.php?discount=". $discountCode ."");
    }
?>

Why does that button when clicked, do nothing but reload the page? I want it to redirect the user to payment.php?

Entire form with the submit button...

<form method="post" action="cart_update.php">
    <div class="row" style="padding-bottom: 10px;">
        <div class="col-md-2">
            <strong>Quantity</strong>
        </div>
        <div class="col-md-3">
            <strong>Name</strong>
        </div>
        <div class="col-md-2">
            <strong>Price</strong>
        </div>
        <div class="col-md-2">
            <strong>Total</strong>
        </div>
        <div class="col-md-3">
            <strong>Remove</strong>
        </div>
    </div>

    <?php
        if(isset($_SESSION["cart_products"])) { // Check session var
            $total = 0; // Set initial total value

            foreach ($_SESSION["cart_products"] as $cart_itm) {
                // Set variables to use in content below
                $ProductName = $cart_itm["ProductName"];
                $product_qty = $cart_itm["product_qty"];
                $Price = $cart_itm["Price"];
                $ProductCode = $cart_itm["ProductCode"];
                $subtotal = ($Price * $product_qty); // Calculate Price x Qty
                echo '<div class="row" style="padding: 10px 0;">';
                echo '<div class="col-md-2">Qty: <input type="text" size="2" maxlength="4" name="product_qty['. $ProductCode .']" value="'. $product_qty .'" /></div>';
                echo '<div class="col-md-3">Product Name: '. $ProductName .'</div>';
                echo '<div class="col-md-2">Price: '. $currency . $Price .'</div>';
                echo '<div class="col-md-2">Total: £';
                echo money_format('%i', $subtotal);             
                echo '</div>';                      
                echo '<div class="col-md-3"><input type="checkbox" name="remove_code[]" value="'. $ProductCode .'" /></div>';
                echo '</div>';
                echo '<hr>';
                $total = ($total + $subtotal); // Add subtotal to total var
                $grand_total = $Price * $product_qty;
            }

            $grand_total = $total + $shipping_cost; // Grand total including shipping cost
            foreach($taxes as $key => $value) { // List and calculate all taxes in array
                $tax_amount = round($total * ($value / 100));
                $tax_item[$key] = $tax_amount;
                $grand_total = $grand_total + $tax_amount;  // Add tax val to grand total
            }

            $list_tax = '';

            foreach($tax_item as $key => $value) { // List all taxes
                $list_tax .= $key. ' : '. $currency . sprintf("%01.2f", $value).'<br />';
            }

            $shipping_cost = ($shipping_cost) ? 'Shipping Cost: ' . $currency . sprintf("%01.2f", $shipping_cost).'<br />':'';
        }
    ?>

    <br />

    <div class="row">
        <span style="float: right; text-align: right; width: 100%;">Amount Payable: <strong>£<?php echo sprintf("%01.2f", $_SESSION['grand_total'] = $grand_total); ?><br />Price inc. VAT and Postage + Packaging</strong><br />Discount Code: <input type="text" name="discountCode" id="discountCode" value="TEST" /></span>

        <br />
        <br />

        <a href="index.php" class="btn btn-warning">Continue Shopping</a>
        <button type="submit" class="btn btn-warning">Update</button>
        <!-- <a href="payment.php" class="btn btn-warning">Purchase</a> -->
        <input type="submit" name="submitOrder" id="submitOrder" value="Purchase" class="btn btn-warning">

        <?php
            if (isset($_POST['submitOrder'])) {
                header ("location:payment.php?discount=". $discountCode ."");
            }
            $_SESSION['ProductName'] = $ProductName;
            $_SESSION['product_qty'] = $product_qty;
            $_SESSION['grand_total'] = $grand_total;
        ?>
    </div>

    <input type="hidden" name="return_url" value="<?php $current_url = urlencode($url="http://".$_SERVER['HTTP_HOST'].$_SERVER['REQUEST_URI']); echo $current_url; ?>" />
</form>

Answer 1

ensure the very first item returned is the header with no whitespace or anything before it ie echos or text,

check the log files for headers already sent errors

make sure the button is inside the form