opencart/opencart — pull request #564 by ADDCreative (2013-03-20 17:50:54+00:00)
Title: Fix CURLOPT_HEADER in pp_standard.php

CURLOPT_HEADER is a bool. Force curl HTTP 1.1 headers as per PayPal's
example.

https://www.x.com/developers/PayPal/documentation-tools/code-sample/216623