# $_ Variables are referred to as Superglobals

$GLOBALS  # Global Variables, think _G in lua
$_SERVER  # Miscellanous information, such as argv and argc!
$_GET     # Get URL Parameters
$_POST    # Variables passed via the POST method
$_FILES   # Files uploaded to the script
$_COOKIE  # HTTP Cookies
$_SESSION # Session Variables
$_REQUEST # Contains request info like $_GET, $_POST and $_COOKIE 
$_ENV     # Environment variables, just as youd expect