[SeanLuke]

> why would you expect it to support ranges larger than 2^32?

Because this is not a simple, stupid linear congruential generator. mt_rand(), is, as I understand it, based on Mersenne Twister, a well-regarded generator which has a period of 2^19937. And if PHP had done this right, mt_rand() would be seedable with an array of some 624 integers.

Mersenne Twister has an alternative "classic" seeding algorithm, drawn from Knuth, to be compatible with old-style generators. This algorithm takes a 32-bit seed. Apparently that's all that PHP is supplying.

It's dead simple to do random(n) uniformly. When I heard that some language implemented a function called mt_rand() which was hilariously broken in this respect, the first thing that came to my mind was "that's gotta be PHP". And it was.