The Phantom of the Opera (the novel) was written by Gaston Leroux. The more famous musical adaptation was composed by Andrew Lloyd Webber with lyrics by Charles Hart and Richard Stilgoe. There hasn't been an actual Opera of it though, so this question is kinda out of place in the opera category, and also Mozart????
He never wrote an opera.
Mozart
Andrew Lloyd Webber wrote this modern opera.
Wolfgang Amadeus Mozart wrote the music for the opera Così fan tutte.
The opera Mefistofele was written by the Italian composer Arrigo Boito.
Gaston Leroux wrote the original book of Phantom of the Opera, but it was later made into a film/play by Andrew Lloyd Webber, who also wrote the music for it.
Ferdinand Herold
Pyotr Tchaikovsky
Giancarlo Menotti.
josplin
The Tender Land He also wrote an opera for performance in schools titled "Let's Make an Opera."
The Italian seria (drama) opera in two acts 'Idomeneo'.