Wow! First of all, welcome to the board. We appreciate all the new members.
As to the Phal, not sure how to respond to this. "Hollow" roots sound dried out (ie. dead) to me. "Wilted" leaves could indicate drout (dryness) or flood (wetness) in pot. Both will end up killing the roots for different reasons. When repotting a Phal with these symptoms, I personally would not recommend sphagnum, not even New Zeland sphagnum which is the best. If the roots are in fact dead, adding lots of moisture holding at the roots won't help the plant.
Here's what I would do if it were mine (keep in mind I am not necessarily suggesting this in your case without seeing the plant.)
1) I would trim off all the roots that look suspect to me, mushy brown, hollow, etc.
2) I would remove the whole flower spike down close to the leaves so it wouldn't try to rebloom.
3) I would sprinkle powdered cinamon all over the remaining roots (if any).
4) I would then place the whole plant in a zip loc bag with a small handful of moistened sphagnum (not in contact with the plant) for moisture. I would NOT water the plant, but WOULD keep it as moist (humid) as possible until I see the first nubbins of roots appear. While this is going on I would keep the plant in the bag next to an east or north window for low to moderate light. I definitely would attempt to keep plant out of sun, but not too dim either.
5) Once new roots appear and the leaves plump back up, I would repot, but in fir bark or Semi Hydroponics (I favor fir bark).
All of the above are known on the board as the "sphag and bag" method and you will see that mentioned from time to time. It is the "intensive care ward" of your grow area.
Let us know how this turn out.