A common digraph in English is 〈ph〉, which represents the sound /f/, and can be used to transliterate 〈φ〉phi in loanwords from Greek. In German, the digraph 〈pf〉 is common, representing a labial affricate/pf/.
Most English words beginning with P are of foreign origin, primarily French, Latin, Greek, and Slavic; these languages preserve Proto-Indo-European initial /*p/. Native English cognates of such words often start with F, since English is a Germanic language and thus has undergone Grimm's law; a native English word with initial /p/ would reflect Proto-Indo-European initial /*b/, which is so rare that its existence as a phoneme is disputed.
However, native English words with non-initial P are quite common; such words can come from either Kluge's law or the sp combination; PIE /*p/ is preserved after s.