code: public static function env()
      {
          return function () {
              $appId = getenv(self::ENV_APP_ID);
              $certId = getenv(self::ENV_CERT_ID);
              $devId = getenv(self::ENV_DEV_ID);
  
              if ($appId && $certId && $devId) {
                  return new Credentials($appId, $certId, $devId);
              } else {
                  return new \InvalidArgumentException(
                      'Could not find environment variable '
                      . 'credentials in '. self::ENV_APP_ID . '/'
                      . self::ENV_CERT_ID . '/'
                      . self::ENV_DEV_ID
                  );
              }
          };
      }
docs: Provider that creates credentials from environment variables
  EBAY_SDK_APP_ID, EBAY_SDK_CERT_ID, and EBAY_SDK_DEV_ID.
  
  @return callable