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. For a listing of these variables, please see the section on Reserved Predefined Variables.
In PHP 4.2.0 and later, the default value for the PHP directive register_globals is off. This is a major change in PHP. Having register_globals off affects the set of predefined variables available in the global scope. For example, to get DOCUMENT_ROOT you'll use $_SERVER['DOCUMENT_ROOT'] instead of $DOCUMENT_ROOT, or $_GET['id'] from the URL http://www.example.com/test.php?id=3 instead of $id, or $_ENV['HOME'] instead of $HOME.
Using the available PHP Reserved Predefined Variables, like the superglobal arrays, is preferred.
From version 4.1.0 onward, PHP provides an additional set of predefined arrays containing variables from the web server (if applicable), the environment, and user input. These new arrays are rather special in that they are automatically global--i.e., automatically available in every scope. For this reason, they are often known as 'autoglobals' or 'superglobals'. (There is no mechanism in PHP for user-defined superglobals.) The superglobals are listed below; however, for a listing of their contents and further discussion on PHP predefined variables and their natures, please see the section Reserved Predefined Variables. Also, you'll notice how the older predefined variables ($HTTP_*_VARS) still exist. As of PHP 5.0.0, the long PHP predefined variable arrays may be disabled with the register_long_arrays directive.
Variable variables: Superglobals cannot be used as variable variables inside functions or class methods.
Note: Even though both the superglobal and HTTP_*_VARS can exist at the same time; they are not identical, so modifying one will not change the other.
If certain variables in variables_order are not set, their appropriate PHP predefined arrays are also left empty.
Contains a reference to every variable which is currently available within the global scope of the script. The keys of this array are the names of the global variables. $GLOBALS has existed since PHP 3.
Variables set by the web server or otherwise directly related to the execution environment of the current script. Analogous to the old $HTTP_SERVER_VARS array (which is still available, but deprecated).
Variables provided to the script via URL query string. Analogous to the old $HTTP_GET_VARS array (which is still available, but deprecated).
Variables provided to the script via HTTP POST. Analogous to the old $HTTP_POST_VARS array (which is still available, but deprecated).
Variables provided to the script via HTTP cookies. Analogous to the old $HTTP_COOKIE_VARS array (which is still available, but deprecated).
Variables provided to the script via HTTP post file uploads. Analogous to the old $HTTP_POST_FILES array (which is still available, but deprecated). See POST method uploads for more information.
Variables provided to the script via the environment. Analogous to the old $HTTP_ENV_VARS array (which is still available, but deprecated).
Variables provided to the script via the GET, POST, and COOKIE input mechanisms, and which therefore cannot be trusted. The presence and order of variable inclusion in this array is defined according to the PHP variables_order configuration directive. This array has no direct analogue in versions of PHP prior to 4.1.0. See also import_request_variables().
Since PHP 4.3.0, FILE information from $_FILES does not exist in $_REQUEST.
Note: When running on the command line , this will not include the argv and argc entries; these are present in the $_SERVER array.
Variables which are currently registered to a script's session. Analogous to the old $HTTP_SESSION_VARS array (which is still available, but deprecated). See the Session handling functions section for more information.