What area of PHP does this cover??? (transcript included)

So I was watching some random interviews and I ran into this one: http://mixergy.com/christian-owens-interview/

During the interview the Chris Owens guy said:

We took orders via PayPal. We used the instant payment notification API to ping back our servers, and then sent out the licenses. It was all done in PHP. We worked in three ways. We worked in one way where they’d give us a list of license codes, and then we’d just assign them to people when a sale was made. And then, at the end of the promotion we’d give the list back. We worked on another one where we’d have a URL. We’d ping that URL and that would send the license back, and then we’d send it out ourselves. And then, we had another one where we’d ping the URL, and they’d send it from their servers along to the customer. It was completely automated. So, even if you’re pinging the URL or you’re drawing from a list, it’s all happening automatically for the user

He mentioned this was all done in PHP, could someone tell me what area of PHP this covers and how I could learn to do something similar (and how difficult it would be)? I think it'd be great practice for things related to databases, I just want to know and understand the use of  the instant payment notification API and how difference licenses were drawn for each customer even though they're purchasing the same product.

Probably this is what you would want to look into:  https://cms.paypal.com/us/cgi-bin/?cmd=_render-content&content_ID=developer/e_howto_api_IntroducingExpressCheckoutDG

thanks for the link