public static function getConnectionString($key)
    {
        Validate::isString($key, 'key');

        self::_init();
        $value = null;

        foreach (self::$_sources as $source) {
            $value = call_user_func_array($source, [$key]);

            if (!empty($value)) {
                break;
            }
        }

        return $value;
    }