Cheapest way (platform/language) to implement a RESTful web API for an iPhone app?

Question

I am developing an iPhone app and would like to create some sort of RESTful API so different users of the app can share information/data. To create a community of sorts.

Say my app is some sort of game, and I want the user to be able to post their highscore on a global leaderboard as well as maintain a list of friends and see their scores. My app is nothing like this but it shows the kind of collective information access I need to implement.

The way I could implement this is to set up a PHP and MySQL server and have a php script that interacts with the database and mediates the requests between the DB and each user on the iPhone, by taking a GET request and returning a JSON string.

Is this a good way to do it? Seems to me like using PHP is a slow way to implement this as opposed to say a compiled language. I could be very wrong though. I am trying to keep my hosting bills down because I plan to release the app for free. I do recognise that an implementation that performs better in terms of CPU cycles and RAM usage (e.g. something compiled written in say C#?) might require more expensive hosting solutions than say a LAMP server so might actually end up being more expensive in terms of $/request.

I also want my implementation to be scalable in the rare case that a lot of people start using the app. Does the usage volume shift the performance/$ ratio towards a different implementation? I.e. if I have 1k request/day it might be cheaper to use PHP+MySQL, but 1M requests/day might make using something else cheaper?

To summarise, how would you implement a (fairly simple) remote database that would be accessed remotely using HTTP(S) in order to minimise hosting bills? What kind of hosting solution and what kind of platform/language?

UPDATE: per Karl's suggestion I tried: Ruby (language) + Sinatra (framework) + Heroku (app hosting) + Amazon S3 (static file hosting). To anyone reading this who might have the same dilemma I had, this setup is amazing: effortlessly scalable (to "infinity"), affordable, easy to use. Thanks Karl!

Can't comment on DB specifics yet because I haven't implemented that yet although for my simple query requirements, CouchDB and MongoDB seem like good choices and they are integrated with Heroku.

Answer 1

Have you considered using Sinatra and hosting it on [Heroku]? This is exactly what Sinatra excels at (REST services). And hosting with Heroku may be free, depending on the amount of data you need to store. Just keep all your supporting files (images, javascript, css) on S3. You'll be in the cloud and flying in no time.

This may not fit with your PHP desires, but honestly, it doesn't get any easier than Sinatra.