Why is the f sound in rhytidiadelfus written ph?
heard as rhytidiadelfus
spelled rhytidiadelphus
Because it is a Greek phi. The word is rhytidiadelphus, from adelphos, brother.
What the ear did
Latin took phi as ph and English kept it, which is why we have Philadelphia, phosphorus and philosophy rather than f in all of them. Adelphos turns up in botanical names whenever a genus is described as a brother to another one, and it always brings its ph with it.
R-H-Y-T-I-D-I-A-D-E-L-P-H-U-S
The full entry for rhytidiadelphus covers the name itself.