src/Google/Client.php
public function fetchAccessTokenWithAssertion(ClientInterface $authHttp = null)
  {
    if (!$this->isUsingApplicationDefaultCredentials()) {
      throw new DomainException(
          'set the JSON service account credentials using'
          . ' Google_Client::setAuthConfig or set the path to your JSON file'
          . ' with the "GOOGLE_APPLICATION_CREDENTIALS" environment variable'
          . ' and call Google_Client::useApplicationDefaultCredentials to'
          . ' refresh a token with assertion.'
      );
    }

    $this->getLogger()->log(
        'info',
        'OAuth2 access token refresh with Signed JWT assertion grants.'
    );

    $credentials = $this->createApplicationDefaultCredentials();

    $httpHandler = HttpHandlerFactory::build($authHttp);
    $creds = $credentials->fetchAuthToken($httpHandler);
    if ($creds && isset($creds['access_token'])) {
      $creds['created'] = time();
      $this->setAccessToken($creds);
    }

    return $creds;
  }
public function fetchAccessTokenWithAssertion(ClientInterface $authHttp = null)
  {
    if (!$this->isUsingApplicationDefaultCredentials()) {
      throw new DomainException(
          'set the JSON service account credentials using'
          . ' Google_Client::setAuthConfig or set the path to your JSON file'
          . ' with the "GOOGLE_APPLICATION_CREDENTIALS" environment variable'
          . ' and call Google_Client::useApplicationDefaultCredentials to'
          . ' refresh a token with assertion.'
      );
    }

    $this->getLogger()->log(
        'info',
        'OAuth2 access token refresh with Signed JWT assertion grants.'
    );

    $credentials = $this->createApplicationDefaultCredentials();

    $httpHandler = HttpHandlerFactory::build($authHttp);
    $creds = $credentials->fetchAuthToken($httpHandler);
    if ($creds && isset($creds['access_token'])) {
      $creds['created'] = time();
      $this->setAccessToken($creds);
    }

    return $creds;
  }
Fetches a fresh access token with a given assertion token.
@param ClientInterface $authHttp optional.
@return array access token