I have a Phat Dac which I use on a Pizero in an Air play project (details here http://frederickvandenbosch.be/?p=1447)
and it works very well.
However I have been unable to route audio from Sonic Pi to the phat dac either on a pizero or on a Pi3. I also used an IQAudio board (not at the same time!) and this works well with SP (although it took quite a lot of development by various people over several months before it worked). I believe the HifiBerry board (which uses the same chip as the phat dac) also works.
Sonic Pi uses jackd to connect audio and as supplied it uses the card setting in the Audio device preferences panel (in the latest Raspbian) to select the appropriate card The Phat Dac is not listed here. However I have tried hacking SP to point to the relevant card slot but without success. I have also tried using qjackctl to connect to the card but it is not happy.
Do you know if it is possible to use the card with Sonic Pi?
Did a manual reinstall and it worked!