This is not directly related to your question, but I'd like to bring up a topic which I feel strongly about and will help in your troubleshooting.
The vast majority of the php coders I've come across use a few coding practices which IMO make the code harder to read and maintain. I'm referring to the unnecessary over use of concatenation and excessively long line lengths.
IMO (and what I was taught), line lengths should be kept below 80 chars as much as reasonably possible but can go over a little (no more than 100) on occasion.
Concatenation (and its accompanying additional quotes) should be used only when it's required or logically and syntactically reasonable.
Adjusting the quoting and vertical/horizontal whitespace will solve both of those problems and make your code more readable and maintainable.
For example, I'd add a little horizontal white space to your bind param statements to line the vars up vertically (i.e., columnar format).
I'd do the same thing with the echo statement that Triple_Nothing used.
$Statement->bindParam(':Quantity', $OrderQuantity, PDO::PARAM_INT);
$Statement->bindParam(':Price', $Order['Price'], PDO::PARAM_INT);
$Statement->bindParam(':StockitemID', $Order['ID'], PDO::PARAM_INT);
$Statement->bindParam(':DeliveryDate', $OrderDelivery, PDO::PARAM_STR);
$Statement->bindParam(':KitchenID', $_SESSION['KitchenID'], PDO::PARAM_INT);
$Statement->bindParam(':UserID', $_SESSION['UserID'], PDO::PARAM_INT);
This makes it much easier to read, maintain and find errors.
# this can be changed to a heredoc, if you wanted
INSERT INTO stock_orders (
ON DUPLICATE KEY UPDATE Quantity = $OrderQuantity;