Here is my problem...I want to make a card game that can run on a web server and serve 'x' number of clients that is mostly platform agnostic on the client end (i.e. Android, IPhone, PC, Linux). I was thinking about writing a web service to act as the game server itself and then have 'apps' around it for individual platforms so that users can play the game on different devices. What are some ways I can structure this? What languages and APIs should I consider? I want to make the clients as thin as possible.

I've got some experience with c++, c#, Javascript and PHP and I limited experience with WCF and ASP.NET. I think the JSON format would be best for a medium but I'm open to ideas. Running it on a Microsoft web server is an option but I would like to explore Linux as well since it is a cheaper solution.

I know this is vague but any input on how to get started would be appreciated.

I think you are already most of the way there. All modern mobile devices have some way of consuming and sending json requests over HTTP. You already know PHP and Javascript so use one of these to make a RESTful web service running on a Linux web server.

It should be possible so that if you published a public API then a third party would be able to develop their own client.

I write mobile apps as a day job and the majority of the popular non trivial apps out there (BBC iPlayer, Facebook, Instagram) are really just just rich front ends for interacting with a web database through web services.

I actually started doing something like that, but I was using python as my web server (using BaseHTTPServer), and it was really easy (running on Linux, Windows, Mac, whatever).

FWIW, I didn't know Python until recently, but ti's a simple language, and json can be read and written with ease, so I'd use that as the server, and use a HTML5/javascript app (check out phaser as a game API layer for HTML5/js)

You could look into a preexisting solution such as Google Play Game Services. I haven't used it for multiplayer before though so I don't know how thin the clients would be.

Thanks for the posts all...I'll resurrect this when I get somewhere.