I suppose it really depends on what the disability is. I am partially deaf, and find that the main concern for universities is that I wouldn't hear a fire alarm, which isn't the case, so its not a problem. If the disability would prevent you from doing the PhD and there is no arrangements the university can make to allow you to do the PhD then, that would prevent the project being offered to you. But I can imagine that it is rare for a disability be a problem, seeing as you have already been to university before.