How to run older and newer web applications, feed them the same traffic and compare output between them?

What I need is a way to clone production traffic, and pass it to two versions of application. Output from older one would be served to users as soon as it's available, and when both outputs are ready comparison would be made, and a diff would be logged as an error.

I know how to clone db, and other attached resources, I know how to clone app itself.

Q: How to implement cloning of traffic, and diff computation? Q: Does this technique have a name?

(Right now we use Apache + PHP, but I would be totally fine with different technologies on "front end", especially if there are some existing solutions already)