Superglobals were introduced in PHP 4.1.0, and are built-in variables that are always available in all scopes.
PHP Global Variables – Superglobals
Some 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:
The next chapters will explain some of the superglobals, and the rest will be explained in later chapters.