Hi,
I installed your script. I uploaded the folder and when I do the Checkout Simulator I have always the same IPN problem:

IPN Result: ERROR: IPN handler did not return array:

My OS is Ubuntu 14.04 64bit.
May you help me, please?
Thnaks for your time.

OK, this could be the PHP version issue. I'm aware that this error message appears on certain php installations but I haven't collected enough information to give you a general solution to solve it. I'll update the page as soon as I find a solution. Sorry about that.