Iphone app?

I was wondering if it is possible to write an fm transmitter into an app? Or is an fm transmitter a piece of hardware that would have to be added to the phone? I think this would be very benifical to the app if its possible cause that makes it very easy to use the iphone app with a car stereo.

The iPhone would have to have an FM transmitter built in, which i highly doubt it does. If it did, I'm sure it would be locked out from third party applications because of FCC regulations.

There are a lot of FM transmitters you can use for the iPhone. Apple makes them but so do alot of other companies. You'll have no trouble with this at all.