Without me reading through all the Worldpay API (and this time of night, you've got no chance
) i'm guessing you would just do a standard dropdown box and the it will be included in the form by its name.
something like this maybe?
Code:
<select name="worldpay_name">
<option value="Val1">Value 1</option>
<option value="Val2">Value 2</option>
</select>
If it has to be a specific name for Worldpay then you can set it. Usually payment processors build the transaction as an XML doc and send it. It could well build the XML from all the objects inside the form. Otherwise you would get the value using:
PHP Code:
$name = $_POST['worldpay_name'];
I know its a bit vague but never used Worldpay. Hope that gives some clues to the API though.