public function callback() {

        $token = required_param('oauth_token', PARAM_RAW);
        $verifier = required_param('oauth_verifier', PARAM_RAW);
        $secret = $this->flickr->get_request_token_secret(['caller' => 'repository_flickr']);

        // Exchange the request token for the access token.
        $accesstoken = $this->flickr->get_access_token($token, $secret, $verifier);

        // Store the access token and the access token secret in the user preferences.
        set_user_preference('repository_flickr_access_token', $accesstoken['oauth_token']);
        set_user_preference('repository_flickr_access_token_secret', $accesstoken['oauth_token_secret']);
    }