问phpExcel TextValueBinder不明白我为什么需要它

EN

function file_to_obj_php_excel($inputFileName)
{
    $CI =& get_instance();      
    if ($CI->config->item('spreadsheet_format') == 'XLSX')
    {
        $objReader = new PHPExcel_Reader_Excel2007();
    }
    else
    {
        $objReader = new PHPExcel_Reader_CSV();
        PHPExcel_Cell::setValueBinder(new TextValueBinder());
    }

    $objReader->setReadDataOnly(true);
    $objPHPExcel = $objReader->load($inputFileName);

    return $objPHPExcel;
}

class TextValueBinder implements PHPExcel_Cell_IValueBinder
{
    public function bindValue(PHPExcel_Cell $cell, $value = null) 
    {
        $cell->setValueExplicit($value, PHPExcel_Cell_DataType::TYPE_STRING);
        return true;
    }
}