Php的create_sid（）任何人都用过吗？

Was wondering how I can use this PHP function, PHP manual

public string SessionHandler::create_sid ( void )

it is clear that it takes none as argument and returns session id, how I can write this( in my custom session handler class)

    public function create_sid()
    {
        // return $id; <--- like this or nothing just need to call this when need to regenerate session id.
    }

PHP manual doesn't say anything clearly on this.