code: public function getServerVersion()
      {
          if ($this->_serverVersion === null) {
              $this->_serverVersion = $this->db->getSlavePdo()->getAttribute(\PDO::ATTR_SERVER_VERSION);
          }
          return $this->_serverVersion;
      }
docs: Returns a server version as a string comparable by [[\version_compare()]].
  @return string server version as a string.
  @since 2.0.14