Which OS are you going to be installing this onto? Linux? Windows 2000/XP/NT?
In order for PHP to work, it should be on the same server as the web server. Otherwise, your network connection between the two machines will be a bottleneck. (the transfer rate between applications in memory is MUCH faster than sending data between two machines)
I agree that mySQL should be placed on a separate server since it will be handling most of the heavy processing tasks (for running queries) As for scalability, you'll be surprised how much a machine running Apache/PHP can handle.