I found the code in a YouTube video, and it worked fine in the video. However, when I enter it into my Atom text editor and display it in my browser, I get a completely blank page. Does anyone know why?
Unless it's different on your version of PHP, the actual value is $_SERVER['DOCUMENT_ROOT'], not in the environment.
It's also considered bad form for general application use, since it references web application resources via their filesystem names (brute force) and not the application server's resource location mechanisms.
I'm not sure of what sort of mayhem that approach can do in PHP, but in Java, accessing a web application resource via a filesystem name can be fatal, as technically, the web application is supposed to be contained within a single WAR file and therefore the application resources do not even have discrete filesystem paths. The only thing that saves most people who do that is that many webapp servers unzip ("explode") WAR files before use. But it's still bad practice.
"privilege" comes from the Latin words for "private" and "law" (legal) and dates to feudal times. To "claim privilege" meant that you were above the laws that applied to the common people.