Descriptionint gmp_prob_prime ( resource a [, int reps])
If this function returns 0, a is definitely not prime. If it returns 1, then a is "probably" prime. If it returns 2, then a is surely prime. Reasonable values of reps vary from 5 to 10 (default being 10); a higher value lowers the probability for a non-prime to pass as a "probable" prime.
The function uses Miller-Rabin's probabilistic test.