PHP provides a large number of predefined variables to any script which it runs. Many of these variables, however, cannot be fully documented as they are dependent upon which server is running, the version and setup of the server, and other factors. Some of these variables will not be available when PHP is run on the command line. Refer to the list of predefined variables for details.
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 rather special in that they are automatically global - i.e., 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.