In PHP, predefined variables are variables that are already built into PHP without the programmer having to declare them or assign anything to them. They can provide a wide variety of different of useful information. $_SERVER is an example of a predefined variable that is an array of variables containing server and execution environment information.
The variable $_SERVER contains 39 elements.
Here is an example of three of them:
'SERVER_NAME': mail.tdebroux.soisweb.uwm.edu
'REQUEST_METHOD': GET
'SERVER_SOFTWARE': Apache