(PHP 4 >= 4.1.0, PHP 5, PHP 7)
$_SESSION -- $HTTP_SESSION_VARS [deprecated] — Session variables
An associative array containing session variables available to the current script. See the Session functions documentation for more information on how this is used.
$HTTP_SESSION_VARS contains the same initial information, but is not a superglobal. (Note that $HTTP_SESSION_VARS and $_SESSION are different variables and that PHP handles them as such)
This is a 'superglobal', or automatic global, variable. This simply means that it is available in all scopes throughout a script. There is no need to do global $variable; to access it within functions or methods.