How to understand Json_decode and covert it to save-able format for database

When I get facebook login token and send it to graph link, it will return a json string, in which I find this type of format:

Luy\u1ec7n

I don't understand how facebook encode it but how it appears on facebook page actually (Vietnamese) is:

Luyện

Then I use:

$array = json_decode($data, true);

It becomes: Luyện

How could I change it to the original word Luyện ? and save it to my database ? and then, is it right way to save name to database then later query from database for other checking purposes ?