While splitting apart a website into discreet applications, I moved all the shared code to its own folder. While that isn't a hard thing to do, what does one do in a development environment where the shared files might be undergoing changes that you don't want other environments to see?
Set a custom include path!
In each of my Virtual Host sections, I added the following line:
<strong>php_value include_path '/path/to/my/include'</strong>
With a restart of Apache, each environment now looks to its own custom include folder.