Many mobile applications are self-contained programs (such as a Calculator app) and have no need to interact with other services or systems. However, will find that as you build more and more, it will start to become necessary to integrate with external vendors and systems in order to keep your users happy. The recent trend towards integrating Facebook "like" buttons and the ability to Tweet from within an app are excellent examples of this.

In this chapter, we are going to be concentrating on talking to a variety of different service providers in a number of common ways, including basic authorization, open authorization, and using a service provider (such as Urban Airship), coupled with some PHP code, to make push notifications work on your iPhone.