Gonzalo Ayuso has a new post to his site showing how you can set up and use environment variables to make different environments for your applications.

If you use a framework such as Symfony2 this problem is solved for you, but if you aren't using any framework you probably need to solve it in one way or another. [...] The solution that I like for this kind of problem [with having different environments] is to use apache's environ variables. We inject the environ variables in the virtual host configuration.

He shows how to add a variable to the VirtualHost section of your Apache config, how to use the getenv function to retrieve its value and how to use it to select a configuration set. This method can also be applied to other kinds of information including settings you may not want to hard-code directly in you app (like possible database credentials).