That's not a "crazy behavior", it's a limitation of the algorithm they use. mt_rand() only has approx. 2^32 possible seeds; why would you expect it to support ranges larger than 2^32?
The best thing to do is to not use mt_rand().
The best thing to do is to not use mt_rand().
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.
Returning garbage when one of your parameters is beyond a certain limit is "crazy behavior." Either support it properly, or don't support it at all.
I don't think a language like PHP, or indeed anything written in it, should have undefined behavior but I'm not the author of the library.
If you expect input parameters in some range and you get values out of that range, then you blow up. You don't silently truncate anything and you certainly don't reduce the entropy of your random number generator by nearly 50%.