Answer:
if you read the prologue of the book it says that he was real. Plus the story actually could be true!
The book is based on old stories of phantom and also facts of a letter requesting money and when the money was given for collection it disappeared also when the opera house was sold many weird things were found deep in the bowels of the house like a mirrored room with a metal hanging tree, many concealed doors and a finished opera which may have been called Don Juan which had never been performed. A white mask was also found.
a few of these 'torture chambers' had been set up around the world but the genius who created them was always kicked out and sent away. Also one of the few people who had seen communicated and concealed the 'Phantom' was visited by him as the phantom was dying.
There was mysterious disappearence of the Viscount de chagny and Christine daae a soprano at the opera house.