Is it legal for doctor to give placebo antibiotics without patient knowing ? does it happen often ?

I don't believe: it is illegal. Not real good medicine. If I don't think antibiotics are needed I will not give them. So I don't believe it happens often.

Never seen it happen: Don't think there's any law about that (and there shouldn't be). Haven't seen a doctor prescribe placebo antibiotics, although a patient might misinterpret a doctor's explanations and end up incorrectly believing he is getting a placebo. Of course, if a patient demands antibiotics when there is no need for antibiotics, and ends up taking them, his very real antibiotics are his placebo.