protected $fallback = 'mysql_fallback';

    public function __construct($host = "mysql", $user = "xxx", $pass = "xxx", $db = "xxx", $port=null, $socket=null) {
        @parent::__construct($host, $user, $pass, $db, $port, $socket);
        if ($this->connect_errno) {
            @parent::__construct($this->fallback, $user, $pass, $db, $port, $socket);
        }
    }

$db = new myDbase();`
if (!$db->connect_errno)`{