Well technically "The Phatom of the Opera" is an opera, as there is no dialogue and it is all sung, but due to the lack of traiditonal recitative/aria, or particularily spohisticated orchestration, it is better suited to the broadway stage, and so it is seen by the public as a musical.
Unless you're talking about Maury Yeston's "Phantom" which is a true musical as opposed to opera, and frankly I think much of his music and telling of the story is far more beautiful than Webber's. Sorry for getting off topic, I could talk about opera/musicals all day.