What is the form or structure of the musical The Phantom of the Opera?

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.