A fan of Phantom of the Opera. Phan was used for this meaning to describe fans of the musical "The Phantom of the Opera" long before it was used as a ship name.
Jess (on first date): Joe, I must warn you that I am a Phan and require that we sing "All I Ask of You" if we want to have a serious relationship.
Joe: OMG, YES! I'm a Phan too!
Phantom Phans, fans of the character Erik (The Phantom) in The Phantom of the Opera. A 'Phangirl' or 'Phan-girl' is a girl Phan and a 'Phanboy' or 'Phan-boy' is a boy Phan.
She's such a Phan-girl she writes PhanPhic about Erik!