I suspect english words starting with 'ph' come from french, french words with 'ph' used 'ph' instead of 'f' (same sound) to emphase the fact the word borrow greek root (but who care the word come from greek, does it make it easier to learn? one more shitty rule of french).
jimmy1|7 years ago
Heuristically, the "rule" passes the eyeball test: philosophy, physical, photo, phrase, philanthropy, phobia, phage, phalange, phalanx, phallic, phase, pharmacy, phantom, phenomenon, phone, photons, photosynthesis, physician, physique, phytoplankton, so on and so forth
The overwhelming majority of these words are greek transliterations https://www.thefreedictionary.com/words-that-start-with-ph
(in fact I am having a hard time finding even one word that isn't -- i think "phreak" is the only one I can find)