Predefined variables are variables that are always accessible when using PHP.
Some variables provide various amounts of information like information about the server or information that was gathered from a form.

The server global variable provides information about the server:
The server name that hosts this PHP file is: hillia35.soisweb.uwm.edu

The server global variable also can provide information about the server's IP address:
The server's IP address is: 129.89.43.13