EDIT:
The links below which apparently were good for 3 years are no longer working so I went and found a couple of new tutorials that I think are going to stick around for a while. These are on the Ray Wenderlich site, a very well respected ios dev tutorial site. The first article actually references the broken links below but it is complete within itself:
How To Write A Simple PHP/MySQL Web Service for an iOS App
and the second one has a little twist to it. It used parse.com on the backend and AFNetworking. Both of which are quite excellent.
How To Synchronize Core Data with a Web Service – Part 1
I have fixed the broken links below by finding the articles in the way back machine. People seem to like the links so I will keep them. The links above should provide more food for thought.
I am doing exactly the same thing with my iphone app. I found this article on building a RESTful API in PHP:
and there is also a followup article here:
with a link to source code at the bottom of the article.