I have the script ready for distribution when the BTC is ready.

Includes:

Dynamic BitcoinPayFlow Library (bitcoinpayflow.php)
BitcoinPayFlow Test (payflowTest.php)
Callback using SHA2 for verification (callback.php)

BTW, their documentation is seriously lacking content.  I have written 100's of API libraries and you always get a clear indication of the API url...

EDIT: Open Source - See sig