How does pfgw evaluates phi(x,y)?

I found this number to be a PrP, but can't write it in a more
palatable way:

Primality testing phi(30,2^4200+13) [N-1/N+1, Brillhart-Lehmer-
Selfridge]
Running N-1 test using base 11
Running N+1 test using discriminant 17, base 12+sqrt(17)
Calling N-1 BLS with factored part 0.29% and helper 0.03% (0.90% proof)
phi(30,2^4200+13) is Fermat and Lucas PRP! (72.3111s+0.0435s)

Lélio