First, if you enter the hall's from the main lobby I'll call it, on your right, the door on the left in the lobby, and proceed up the hall, opening the hidden door, you will see a mob in the corner. Turn right and there is another hidden door on your left, but also your right. Left goes towards Grounds, right puts you in a little tiny room where 2 mobs spawn. 1 of these spawns is a PH. If you go too far you will be heading downstairs, come back a tad and try again.
Now, go back out, and straight towards the Grounds Zoneline. As you open this hidden door and the regular one behind it, you will notice 1 soldier mob standing in front of you. He is also a PH. Behind him is a hidden false wall. There is a smaller Nightmare soldier style mob here. He will add if you pull this one, so be careful.
These are the only 2 PH's I know of. Well 3 technically.
Hope that helps.