I am new to web development and server set up. I am looking for some advice or a link to a tutorial on setting up a production system up. Right now, I have a server (Ubuntu, Apache, MongoDB, and PHP). It receives a request, PHP queries Mongo, and PHP sends out the requested data. How do I make that work with more servers? I've read that you can make a cluster of a primary and two slave nodes which work as separate servers running Mongo, but do those also run PHP? Or is the primary the only one running the PHP? I have read some docs on Mongo site and a video of someone from 10gen going through it, but they are geared towards people that seem to already understand this stuff, I have no idea and need to start from a beginning stage.

If anyone can help me understand where PHP (Acting as my API) lives in these clusters, that would be greatly appreciated! Thanks in advance for any help!