There are a handful of predefined variables in PHP. They include:

$GLOBALS — References all variables available in global scope

$_SERVER — Server and execution environment information

$_GET — HTTP GET variables

$_POST — HTTP POST variables

$_FILES — HTTP File Upload variables

$_REQUEST — HTTP Request variables

$_SESSION — Session variables

$_ENV — Environment variables

$_COOKIE — HTTP Cookies

$php_errormsg — The previous error message

$HTTP_RAW_POST_DATA — Raw POST data

$http_response_header — HTTP response headers

$argc — The number of arguments passed to script

$argv — Array of arguments passed to script

The most commonly used ones, in my opinion, are $_GET, $_POST, $_SERVER, $_FILES, $_SESSION and $_COOKIE (so, the majority of them!)