Generates a random deviate
(PECL stats:1.0.0-1.0.2)
float stats_rand_gen_f ( floatdfn, floatdfd )
Generates a random deviate from the F (variance ratio) distribution with
"dfn" degrees of freedom in the numerator and "dfd" degrees of freedom in
the denominator. Method : directly generates ratio of chisquare variates.