That's not how it works. Say for the sake of argument there's a 0.1% chance of a drop, and mt_getrandmax() is 2147483647. You call mt_rand() and if the result is < (2147483647/1000) then a girl drops. If you want to increase the chance of a drop you don't go "oh mt_rand() returned 9823625 I'll remember that number and if mt_rand() returns that again I'll get a new random number instead" you just increase the probability.