This page would actually become the confirmation page. You would make all of the form elements hidden and then create another checkout page that has the visible form that is filled out.
Then populate the hidden form on this page with the values from the form on the previous page so that they are automatically the values in this form which you already have working.
PS although your code may work, the code would more properly be:
$PP_DirectPayment_itemized = "".((isset($_POST["reference"]))?$_POST["reference"]:"") ."";