Had a chance to play around with what used to be Amazon Web Services and is now Amazon ECS. The previous times I've used web services with PHP (or C# for that matter) I've used SOAP to access the data. I had heard of REST but didn't really know what it was and assumed SOAP was better.
I think I was wrong. REST requires no additional libraries and accomplishes everything it needs to using URLs. The return value for any query is a "page" that's really just an XML document. From there you can parse it how you want or use XSL to format it.