PHP provides a large number of predefined variables to any script that it runs. These variables are superglobals, which are available anywhere in a script. Here's a list of the primary ones:
$_GLOBALS. Contains every variable that is currently available within the global scope of the script. The keys of this array are the names of the global variables.
$_SERVER. Variables set by the web server or otherwise directly related to the execution environment of the current script.
$_GET. Variables provided to the script via HTTP GET.
$_POST. Variables provided to the script via HTTP POST.
$_COOKIE. Variables provided to the script via HTTP cookies.
$_FILES. Variables provided to the script via HTTP POST file uploads.
$_ENV. Variables provided to the script via the environment.
$_REQUEST. Variables provided to the script via the GET, POST, and COOKIE input mechanisms.
$_SESSION. Variables that are currently registered to a script's session.