Format::setUnderline

require_once 'Spreadsheet/Excel/Writer.php';

  $workbook = new Spreadsheet_Excel_Writer();
  $format_underline =& $workbook->addFormat();
  $format_underline->setUnderline(1);
  $worksheet =& $workbook->addWorksheet();
  $worksheet->write(0, 0, "This is underlined", $format_underline);
  $format_underline->setUnderline(2);
  $worksheet->write(1, 0, "This is twice underlined", $format_underline);