I have a requirement from a client for my PHP Web application to be 3-tier. For example, I would have a web server on Apache in the DMZ, but it should NOT contain any DB connections. It should connect to a Middle server that would host the business objects but be behind the firewall. Then those objects connect to my SQL cluster on another server.

I have actually done this using .NET, but I am not sure how to setup my stack using PHP. I suppose I could have my UI front tier call the middle tier using REST based web services if I create my middle tier as a second web server, but this seems overly complex.

The main reason for this is advanced security: we can not have any passwords on the DMZ first tier web server. The second reason is scalability - to have multiple server on different tiers that can handle the requests. The Last reason is for deployment - it is easier if I can take one set of servers offline for testing before putting them back in production.

Is there a open source project that shows how to do this? The only example I can find is the web server hosting files from a shared drive on another machine (kind of how DotNetNuke pretends to be 3-tier), but that is NOT secure.