When this occurs in onset position, it indicates Ancient Greek etymology, a transliteration of the Greek letter psi and is pronounced as an unvoiced alveolar sibilant /s/ sound, i.e. the P is a silent P. For example, pseudoscience, psychology, etc.
When this letter combination appears in coda, it's typically P with plural -s (e.g. the stops), or P with third person -s (e.g. it stops) so is not technically a digraph as they are parts of distinct morphemes.