No, it cannot be an old spelling, since it comes from Greek: an initial "di-" (δι- from dual, two) and then phthong (φθόγγος, voice). So the two initial letters

No, it cannot be an old spelling, since it comes from Greek: an initial "di-" (δι- from dual, two) and then phthong (φθόγγος, voice). So the two initial letters of the second part are phi and theta.