One of my favorite aspects of the English language is that not only is it shameless about stealing other languages' words, but it seems to take it as a point of pride. It's a good feature to have if you want something global- it means that useful words will accumulate without regard to origin, which benefits everyone.
"We don't just borrow words; on occasion, English has pursued other languages down alleyways to beat them unconscious and rifle their pockets for new vocabulary."
and also by extension, Greek (directly in many cases -- almost every english word that starts with 'ph' was a greek word -- but also a large portion of medical and mathematical terminology)
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).
Granted I don't have hard data on the "ph" fact, the source is pretty credible in my opinion. The fact appeared in an article written for the British Council, co-authored by Martha Peraki, a Linguistics PhD recipient from American University.
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
