Hey, you forgot to talk about Wddx in your article.
While WDDX is in between XML-RPC and SOAP, it's probably closer to JSON. It allows serializing of values nicely in PHP, without breaking anything.

Yes, I know, Wddx is mainly made for coldfusion, but it's just a standard. OpenWddx says you can use Wddx without problems, and I personally like it better than XML-RPC and SOAP when it comes to communications between a webserver and a clientside application.