"Ph" for the /f/ sound; Is Old English responsible for this swap?

Is Old English responsible for creating the /f/ sound from ph, as in Philip, Pharoah, Physics, Sophia, etc? Many European countries keep the f for all of their /f/-sounding letters, as in Sofia and Stefan, for example.

Old English is definitely not responsible for this.

All of the words that you mentioned are Greek in origin, and they all contained the Greek letter φ (phi). In Classical Greek this was pronounced as an aspirated [pʰ], which the Latins wrote as ph when they borrowed the words from Greek. Later this sound changed into an [f] in both Greek and Latin, and was passed as such into French, and then into English.

Once the idea that ph was pronounced [f] was established, it spread to a few other areas, as well. Borrowings from Hebrew and other Semitic languages sometimes use ph, especially since the Hebrew letter פ can be [p] or [f] depending on context. Vietnamese regularly uses ph for [f], in this case because the modern Vietnamese orthography was designed by the French.

There were two p, t, and k sounds in ancient Greek. The softer (aspirated) sounds were transliterated in Latin as ph, th, and ch. Then, in Greek, all three sounds weakened; respectively they sounded like f, th (as in think), and the soft throaty sound in German ich or the x in Spanish Mexico. Neither Greek nor Latin changed the way the sounds were written. More than 1000 years later, English was born and then written down by people trained in Latin; they kept the Latin conventions for transliterating Greek words. You can read a more in-depth explanation about the history of these spellings at tellingvoice.com.

The Romans. They translated Greek phi (φ) as "ph" and pronounced it closer to "p" than "f". Native Latin words were spelled with the "f". Ironically, a lot of Latin p-words became f-words (e.g., pedis became "foot").