Why PhD, and not DPh
Solution 1:
Ph.D. is an abbreviation of the Latin, not the English.
Wikipedia says:
A Doctor of Philosophy degree (often abbreviated Ph.D., PhD, D.Phil., or DPhil) or a Doctorate of Philosophy, from the Latin Doctor Philosophiae...
Latin does not have a set word order in cases like this, so Philosophiae Doctor is perfectly legitimate as well, and that's what the abbreviation is based on.