Casual Conversation

I cannot think of any language besides English in which an “f” can be written as “ph”.

Latin. In fact it's where this mess started out.

Ancient Greek had a three-way distinction between the following sets of consonants:

• ⟨Φ Θ Χ⟩ /pʰ tʰ kʰ/ - they sound like English pill, till, kill; there's a clear asphhhhiration in them
• ⟨Π Τ Κ⟩ /p t k/ - they sound like English spill, still, skill; no aspiration
• ⟨Β Δ Γ⟩ /b d g/ - more like English bill, dill, give; instead of aspiration you vibrate the vocal folds before the consonant even starts

Latin borrowed a lot of Greek words. The words with the second and third set of consonants were no problem; they were mostly spelled in Latin with ⟨P T C⟩ and ⟨B D G⟩. But Latin didn't have the sounds of the first set, and for Latin speaking ears they sounded like they had /h/. So they were spelled with ⟨PH TH CH⟩, to represent that /h/ sound.

So back then the digraphs still made sense... except that Greek changed over time. And what used to be pronounced /pʰ tʰ kʰ/ ended as /f θ x/ (like English fill, think, and Scottish loch). And Latin speakers started pronouncing those words with the "new" Greek sounds instead of the old ones. But they were still spelling them the same.

From that that ⟨PH⟩ spread out across a lot of orthographies using the Latin alphabet.