PHP artist tags stand for "Person(s) Hiding Ponies". They are used when an artist wishes to anonymise themselves after-the-fact, without resorting to taking down all of their work.
They're pretty much an "I don't want to delete my art but I want to try and untie my name from it" thing.
As privacy is one of our priorities, please do not publicly state which artist is associated with which PHP tag. Thanks!