"Always exercise caution in trusting $ENV variables though. Some are wholly unreliable much of the time, while others can be easily spoofed by a custom script trying to behave like a browser."

Correct me if I'm wrong, but a DOCUMENT_ROOT is quite hard to spoof client side, since it is a server side variable, unlike for example HTTP_USER_AGENT. In the latter case, the script relies on (l)user input, and thus it's easy to spoof. DOCUMENT_ROOT is specified by the http daemon, not the client.

"That means that you could be exposing yourself to a security issue by passing 'chdir' data directly from an environment variable."

In this specific case, I doubt you're exposing yourself to a security risk.