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 consonant cluster /sp/ (PIE *p has been preserved after s).
𐌐 : Old Italic and Old Latin P, which derives from Greek Pi, and is the ancestor of modern Latin P. The Roman P had this form (𐌐) on coins and inscriptions until the reign of Claudius, ca. 50 AD (See also Claudian letters).
𐍀 : Gothic letter pertra/pairþa, which derives from Greek Pi
П п : Cyrillic letter Pe, which also derives from Pi