I guess we can talk about the 'secret' level now.
Purely from (my very bad) memory:
Start by going to O&E. Go up on the elevator type thingies. Go to the right. Go up. Go to the right.
You'll come to a chamber that has a tombstone in it. Throw a fireseed into the right wall and go into the chamber that will open up (takes about 4 fire seeds). There are some steps cut in the wall. Stand in front of the steps and jump up to activate the hidden passage door. You'll end up in another chamber - go to the left, jump down the tunnel. That'll take you to PH.
That's about all I can remember. (haven't played for a while due to it not playing well in Classic and not having a purely OS9 machine hooked up)