code: private function convertError($errorCode)
      {
          switch ($errorCode) {
              case '#NULL!':
                  return pack('C', 0x00);
              case '#DIV/0!':
                  return pack('C', 0x07);
              case '#VALUE!':
                  return pack('C', 0x0F);
              case '#REF!':
                  return pack('C', 0x17);
              case '#NAME?':
                  return pack('C', 0x1D);
              case '#NUM!':
                  return pack('C', 0x24);
              case '#N/A':
                  return pack('C', 0x2A);
          }
  
          return pack('C', 0xFF);
      }
docs: Convert an error code to a ptgErr.
  
  @param string $errorCode The error code for conversion to its ptg value
  
  @return string The error code ptgErr