I wish I knew, I will follow this thread.

Now, my questions are:
How are the roots? (since the leaves are all dehydrated)
When was last time this plant had active root activity?
Is it not following its normal growing cycle?
How do the underside of these leaves look?

I do not think this is a mite damage.
I think I have one phal suffering mite attack, and it starts out by exuding sugar beads on the underside of the leaves, over the course of a few weeks, the number of these tiny clear beads increases.
Then small area of discoloration shows up here and there on the both sides of the leaves.
I looked at the leaf surface with an eye piece and do not see any bugs. It is very frustrating!!!

I have a few phals that display some dark area like yours, but a few small pecks here and there on the top of leaves, not highly localized and concentrated like the one shown up on your phal.
then the tan colored area on the center of this dark area....very interesting.
Can you check for virus?

My suspects are:
1. Fungus, like fusarium wilt?
2. Virus of some sort
3. just crazy physiological defect

Poor, but some thin arial roots.
Over the summer, but very little and only in arial roots, none of which grew by much.
If keeps putting out leaves, not really on a cycle, just always growing a new one, they grow slow and are tiny.Undersides are fine.

Interesting to know how it occurs in Phals. I only really wondered because I know there have been mites around, but they have not been on plants near this one. So I think you are right that it's probably not that.

I don't really know where to get hold of virus checking kit, I think I'm going to have to force myself to chuck this one just in case it's that or a bad fungus :(

Root issues. A plant gets root issues which can affect just a few leaves at first but in the long run the plant is going downhill. I would get a fungal systemic, then repot into better media, keep it warm and bright, then wait. Water only when dry. Phals can grow for a long long time. Nothing limits their growth. But they need a good environment. I would also cut off the damaged leaves, dab on cinnamon, and repot.