The letter f and the digraph ph represent the same sound.
Both the letter f and the digraph ph represent the sound /f/, the voiceless labiodental fricative.
f: fake, fire, fun;
ph: pharmacist, photography, physics.
To pronounce the digraph ph the articulation is the same as the sound f. They are pronounced by impeding the flow of air with the teeth, which come in contact with the lower lip, so a friction sound is produced.