Warning: Undefined array key "HTTP_REFERER" in /home/river222/public_html/infost440/a3/predefined.php on line 14

In the above example, I used $_SERVER['HTTP_REFERER'] and it returned the URL for my a3 index page.

mail.river222.soisweb.uwm.edu
In the above example, I used $_SERVER['SERVER_NAME'] and it returned the server name which is my soisweb URL.

Predefined variables in PHP are "built-in" variables that are already reserved in PHP and allow for quick user access for requested information. They are also called superglobals and will execute based on the predefinition.