ph is usually pronounced /f/ in English. The sound /f/ is made using the upper teeth and the lower lip (hence labio-dental) and not quite bringing them together, thus creating turbulence in the air flowing between them (hence fricative). The vocal cords are not vibrating (so the sound is unvoiced (/v/ is pronounced in the same way as /f/ but with the vocal cords vibrating, i.e., /v/ is voiced)). /f/ is therefore an unvoiced labio-dental fricative.
A plosive (also called a stop) is a sound made by temporarily stopping the flow of air altogether. An unvoiced bilabial (two lips) plosive would be a /p/, while a voiced bilabial plosive would be a /b/.
Some sounds are aspirates. There's a tiny puff of air blown out. The difference between aspirated and unaspirated is not significant for meaning in English, and is not shown in the spelling system. It does occur in, for example, the difference between the pronunciation of /p/ (e.g., pit)at the beginning of a syllable and /p/ at end of a syallable (e.g., tip). In some languages this difference makes the difference between different letter sounds (I think it's true of some Indian languages, come in please, Avy). We're not very good at hearing or producing the difference, just as Japanese have problems with /l/ and /r/, which to us are blindingly obviously different.
Phi and pi were the aspirated and unaspirated versions of an unvoiced bilabial plosive. I will now go and lie down for a while.
Bingley
