code: public function fetchAccessToken($code)
      {
          $this->validateTokenParams();
  
          $this->oauth2->tokenUrl = $this->tokenUrl;
          $this->oauth2->clientId = $this->clientId;
          $this->oauth2->clientSecret = $this->clientSecret;
  
          try {
              $response = $this->oauth2->fetchAccessToken($code);
          } catch (Exception $ex) {
              throw new Oauth2ClientException($ex->getMessage());
          }
          return $this->handleTokenResponse($response);
      }
docs: Fetches the access token using the authorization code
  @author Adegoke Obasa <goke@cottacush.com>
  @param $code
  @return mixed Access token
  @throws Oauth2ClientException