I don't really know the answer so this is a bump
Having said that, my one and only Phag is potted in a mixture of Rock Wool cubes and Bark. That's what it came in from the nursary and they said that the Rock Wool stays damp a lot longer and that Phrags like plenty of moisture (although like all orchids needs a good breathing medium).
I've not had mine long though (end of July) and I'm still pretty frightend of it. I keep it in my living room on the mantle piece because it's still in flower (since I bought it). It probably needs more light than it's getting in the middle of the room, but it's not showing ill effects.