Superglobals were introduced in PHP 4.1.0, and are built-in variables that are always available in all scopes.
PHP Global Variables - Superglobals
Several predefined variables in PHP are "superglobals", which means that they are always accessible, regardless of scope - and you can access them from any function, class or file without having to do anything special.
The PHP superglobal variables are:
- $GLOBALS
 - $_SERVER
 - $_REQUEST
 - $_POST
 - $_GET
 - $_FILES
 - $_ENV
 - $_COOKIE
 - $_SESSION
 
This chapter will explain some of the superglobals, and the rest will be explained in later chapters.