I can't really address this issue in terms of the music, but I can give you some information that might help in terms of form or structure in a literary sense.
You don't specify whether you're referring to the play or the movie, so my answers will be based on the movie, although they may apply to the play as well.
The movie begins and ends with a single red rose. This provides unity and poignancy (a certain quality of specialness). You could say that the structure is circular, then, it ends where it begins. Unity is also provided by the "music box," if that's what you'd call it, which is purchased at the auction at the beginning, and listened to by the phantom at the end.
Musical repetition also creates the structure, once again establishing unity. Songs are repeated often, sometimes with variations in the lyrics.
Repetition is used in other instances as well:
- the phantom's mask is removed twice, once fairly early and once much later
- the phantom hangs someone at least three times in the play
- the idea of masks is prevalent the entire work
- the work moves the audience to the graveyard twice, including in the conclusion
Again, musicals are not my specialty, but looking at Phantom from a literary point of view, I hope some of the above helps.
"The Phantom of the Opera" is performed in musical style. It moves through transitions of opera and rock and roll. It is witty and lyrical with romantic lyrics.
Throughout the movie the music alternates and resonates a similar tone and theme. It is similar to the reuse of the tunes through-out Evita.
The operettas are sung by talented sopranos and are ofset by the deep voice of the phantom whose style is more rock and roll.