I'm having a hard time getting support from from them. the method of comunication (direct quote from their docs is:
"Data will be transferred to [server] via an HTTP POST operation using the MIME type, text/xml"
They want you to mimic what a browser would send to a web site when the user clicks a "send" button in a form.
You use PHP to create the header for the page request, but you need to study HTTP documentation to understand what you need to send. It seems very unreasonable of them to have not supplied sample code.
Good luck.
Stephen