That looks like a simple attempt on doing split testing.
I.e. testing what version of a page converts best.
Reason for that is simply due to if your "optimistic enough" to try and make a load balancer in PHP, you would not base it on modulus, since that can send all visitors through the day to one of the servers if your unlucky.
Still an odd way of doing it, since split testing is normally done with round robin, or geo location.