This is the setup I would go for:
- 2 x PHP/apache servers (if you can/know how to you can also use nginx instead of apache)
- 1 x MySQL server
For running a PHP / MySQL setup, I recommend using virtual servers rather than renting the hardware yourself.
Both Amazon EC2 and Linode servers will do good. The advantages of EC2 is that it's very easy to start new instances if you need them, they have a built in load balancer which you can use and you can make amazon add servers depending on the load. But amazon is a bit expensive, so Linode might be a better option.