public static function constants()
    {
        $class = get_called_class();
        if (!isset(self::$__constants[$class])) {
            $reflection = new ReflectionClass($class);
            self::$__constants[$class] = array_change_key_case(
                $reflection->getConstants(),
                CASE_UPPER
            );
        }
        
        return self::$__constants[$class];
    }