Why does my Submit button not redirect - php

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'] = $grand_total); ?><br />Price inc. VAT and Postage + Packaging</strong><br />Discount Code: <input type="text" name="discountCode" id="discountCode" value="TEST" /></span>
<br />
<br />
Continue Shopping
<button type="submit" class="btn btn-warning">Update</button>
<!-- Purchase -->
<input type="submit" name="submitOrder" id="submitOrder" value="Purchase" class="btn btn-warning">
<?php
if (isset($_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

The problem is, there is output, before you send the redirect headers, because your condition is in the html section.
You should place the code with the redirect before any output is sent to the client.
An other option would be to turn on output buffering, but this must also be done, before something is sent to the client.

Answer 2

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