You can definitely shoot a movie with an iPhone. If it captures footage, you can shoot a movie with it. However, you might be able to achieve some of the visuals you get from a cinema camera, with its larger sensor, wider dynamic range, and better lenses. For instance, it’s hard to achieve tight depth of field using an iPhone. But in the end, the best camera is the one you’ve got on you, and if an iPhone’s all you’ve got, then go for it!