Predefined Variables

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.
From version 4.1.0 onward, PHP provides an additional set of predefined arrays containing variables from the web server (if applicable), the environment, and user input.

These new arrays are rather special in that they are automatically global--i.e., automatically available in every scope.

They are often known as "superglobals".

Related concepts

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Predefined Variables