Predefined Variables

PHP provides a number of predefined variables. PHP also provides an additional set of predefined arrays containing variables from the web server (if applicable), the environment, and user input. These arrays are automatically available in every scope. For this reason, they are often known as "superglobals". (There is no mechanism in PHP for user-defined superglobals.) Refer to the list of superglobals for details.

Note: Variable variables

Superglobals cannot be used as variable variables inside functions or class methods.

If certain variables in variables_order are not set, their appropriate PHP predefined arrays are also left empty.