I'd guess, that this is not possible, as some 200 + views and no answer of yet.
I would have thought that this would be a relatively simple question, being, using APPLE products, but alas, it would seem NOT-SO.
If it was a "simple question"…then you would have already known the answer…wouldn't you??. Or the answer would be very easy to find via a Google search…right??
No it's not a "simple question"…and doesn't have an easy answer! I think that many people reading your question would like to think that it was possible to us an iPhone as a webcam (so they didn't want to immediately say no). But the bottom line is…does an iPhone have a video out port…no it doesn't. And a video out port is probably needed.
Maybe this question falls into the same category as asking…"Can I ride my bicycle to the moon?"