If you want to use your iPhone with Arduino, our friend Pablo Murillo has made a great review about iArd.
Once you download the program in our iOs device, you should configure the IP address of our Arduino. This IP address is provided by the sketch of the developer.
You can download the sketch here.
In the video, we can see how to vary the intensity of the light from a LED connected to the PWM output 9 of Arduino.
In the second example, we have used the iArd readings from the Arduino to convert them in values from 0 to 179 (degrees of the servo).
Via and Source: Arduino Academy