Generates a random deviate from the beta distribution
説明
floatstats_rand_gen_beta(float$a, float$b)
Returns a random deviate from the beta distribution with parameters A and
B. The density of the beta is x^(a-1) * (1-x)^(b-1) / B(a,b) for 0 < x
<. Method R. C. H. Cheng.