UGH!! I don't know where to begin?!

I am trying to build an interface between my website and the payment gateway "Internet Secure". They send and receive data using XML - which I have never used.

They gave me some sample ASP code, but that doesn't help much with PHP?!

I have lots of questions, but I guess the first is...

What should I do with the data I collect in my Payment Form?

Should I first stick it into an array?

Or do I jump right into building an XML string?