The use of "ph" in English to represent the "f" sound originates from the transliteration of the Greek letter "phi" (φ). 
This convention was established when Greek words were adopted into Latin and subsequently into English.

Inglish uses -f- or -ff

Examples:
    philosophy: From Greek "φιλοσοφία" (philosophia), meaning "love of wisdom."
    physics: From Greek "φυσική" (physikē), meaning "nature."
    photograph: From Greek "φωτογραφία" (photographia), meaning "drawing with light."

Inglish:
    filósofy
    fisics
    fôtograff