where is the $_GET['referer'] from？

[linjuming]

when using the palpay method, it use the before_process method , it will check the $_GET['referer'] .but I can not find this nowhere.
anyone can tell me where is it defined?

[lat9]

Using my admin's Tools->Developers Tool Kit and searching for referer in all catalog files produced this result:

C:/xampp/htdocs/v_150/includes/modules/payment/paypal.php

Line #201 : 'return' => zen_href_link(FILENAME_CHECKOUT_PROCESS, 'referer=paypal', 'SSL'),

Line #328 : if (isset($_GET['referer']) && $_GET['referer'] == 'paypal') {

[DrByte]

You should be using Express Checkout instead of IPN/WebsitePaymentsStandard.