I agree with everyone on the wireless keypad. However if you really want to use your iPhone there is a way. I can do it as part of an overall home automation system. It will be quite costly, especially if that is the only function you want.

Goto Homeseer. You will need a windows pc running the software 24/7, some hardware interfaces, like X10 or similar wired to your garage door button. They have an iPhone app that you can customize to control whatever is on your system.

Again, I have all that after years of building it up. I can open the garage door with my phone from anywhere in the world. But after biking, I get off my bike and use the cheapy wireless garage door keypad.