PHP includes many predefined variables that may be used as global variables across the script. 

List of PHP predefined variables 
NAME DESCRIPTION
$_COOKIE Sets or reads cookies
$_ENV Sets or reads environmental variables
$_FILES Reads and manipulates uploaded file's information
$_GET Reads and manipulates uploaded GET variables
$_POST Reads and manipulates uploaded POST data
$_REQUEST Sets or reads REQUEST variables
$_SESSION Sets or reads sessions
$_SERVER Returns information about server
$argc Returns number of arguments passed to script
$argv Returns array of arguments passed to script
$GLOBALS Reads and manipulates all global variables
$HTTP_RAW_POST_DATA Reads and manipulates uploaded raw POST date
$http_response_header Returns HTTP response headers
$php_errormsg Returns error message