code: protected function getActiveVersionFor($messageName)
      {
          $found = null;
  
          if (isset($this->messagesAndVersions[$messageName]) &&
              isset($this->messagesAndVersions[$messageName]['version'])
          ) {
              $found = $this->messagesAndVersions[$messageName]['version'];
          }
  
          return $found;
      }
docs: Get the version number active in the WSDL for the given message
  
  @param string $messageName
  @return float|string|null