code: protected function normalizeIdentifier($identifier)
      {
          if (filter_var($identifier, FILTER_VALIDATE_EMAIL)) {
              return strtolower($identifier);
          } else {
              return self::normalizeOpenId($identifier);
          }
      }
docs: Normalizes the identifier (E-mail address or OpenID)
  
  @param string $identifier E-Mail address or OpenID
  
  @return string Normalized identifier