I've heard 'pie-oh' solver, 'pee-oh' solver, and 'pee-eye-oh' solver.

I spoke to the guy who wrote the solver a ways back. His name is "Piotr" and he's Polish. I would think he named the solver after himself (I did not ask). I sure as hell would. His buddy wrote the graphical interface.

https://en.wiktionary.org/wiki/Piotr
^ see for audio link of pronunciation

If it's pronounced similarly to piotr, then it should be 'pee-oh' solver.