src/Core/OAuth/OAuth2/OAuth2LoginHelper.php
public function refreshToken(){
       $refreshToken = $this->getAccessToken()->getRefreshToken();
       $http_header = $this->constructRefreshTokenHeader();
       $requestBody = $this->constructRefreshTokenBody($refreshToken);
       $intuitResponse = $this->curlHttpClient->makeAPICall(CoreConstants::OAUTH2_TOKEN_ENDPOINT_URL, CoreConstants::HTTP_POST, $http_header, $requestBody, null, true);
       $this->faultHandler = $intuitResponse->getFaultHandler();
       if($this->faultHandler) {
          throw new ServiceException("Refresh OAuth 2 Access token with Refresh Token failed. Body: [" . $this->faultHandler->getResponseBody() . "].", $this->faultHandler->getHttpStatusCode());
       }else{
          $this->faultHandler = false;
          $this->oauth2AccessToken = $this->parseNewAccessTokenFromResponse($intuitResponse->getBody());
          return $this->getAccessToken();
       }
    }
public function refreshToken(){
       $refreshToken = $this->getAccessToken()->getRefreshToken();
       $http_header = $this->constructRefreshTokenHeader();
       $requestBody = $this->constructRefreshTokenBody($refreshToken);
       $intuitResponse = $this->curlHttpClient->makeAPICall(CoreConstants::OAUTH2_TOKEN_ENDPOINT_URL, CoreConstants::HTTP_POST, $http_header, $requestBody, null, true);
       $this->faultHandler = $intuitResponse->getFaultHandler();
       if($this->faultHandler) {
          throw new ServiceException("Refresh OAuth 2 Access token with Refresh Token failed. Body: [" . $this->faultHandler->getResponseBody() . "].", $this->faultHandler->getHttpStatusCode());
       }else{
          $this->faultHandler = false;
          $this->oauth2AccessToken = $this->parseNewAccessTokenFromResponse($intuitResponse->getBody());
          return $this->getAccessToken();
       }
    }
Get a new access token based on the refresh token
@return OAuth2AccessToken     A new OAuth2AccessToken that contains all the information(accessTokenkey, RefreshTokenKey, ClientID, and ClientSecret, Expiration Time, etc)