Difference between predefined PHP $HTTP_SESSION_VARS and $_SESSION session variables
$_SESSION variable is an associative array hold session variable available to the current script. This is automatic global variable also called super global variable. By default, this variable is available in all scopes throughout the script.
Despite of $_SESSION variable, $HTTP_SESSION_VARS variable is not by default auto global but hold same information as $_SESSION variable. register_globals directive is required to set in php.ini file to make this variable available throughout the script.