php เอ็กเซล การ export file to excel ให้อ่าน font ภาษาไทย (Thai language) บน server

<?
ob_start();
session_start();
require( "../initial.inc.php" );

		mysql_query("SET character_set_results=utf-8");
		//mysql_query("SET collation_connection = UTF8-UNICODE-CI", $this->MyConn);
		mysql_query("SET NAMES 'utf-8'");
		
require_once (SECOUND_PATH . CLASS_PATH . "class.writeexcel_workbook.inc.php");
require_once (SECOUND_PATH . CLASS_PATH . "class.writeexcel_worksheet.inc.php");

$token = md5( uniqid(rand(), true ) );
$fname= "../tmp/$token.xls";
$workbook =& new writeexcel_workbook($fname);

$worksheet =& $workbook->addworksheet("customer");

$header =& $workbook->addformat();
$header->set_font("tahoma");
$header->set_bold();
$header->set_align('center');
$header->set_size(12);
$header->set_color('navy');
$header->set_fg_color('silver');

# fix Row 1 
$worksheet->freeze_panes(1, 0);
		
$StrSql = "select C.`CustCode`, C.`CompanyName_EN`, C.`CompanyName_TH`, C.`CustPhone`
			from $G_table C
			LEFT JOIN business_type B ON B.BTypeID = C.BTypeID 
			LEFT JOIN customer_type T ON T.CTypeID = C.CTypeID 
			 $_SESSION[CUSTCond] 
			order by C.`CustCode` asc";
$result=mysql_query($StrSql) or die(mysql_error() . ": " . $StrSql);
	

$worksheet->write(0, 0, "Code", $header);
$worksheet->write(0, 1, "Company name(EN)", $header);
$worksheet->write(0, 2, "Company name(TH)", $header);
$worksheet->write(0, 3, "Phone", $header);

$worksheet->set_column('A:B', 5);
$worksheet->set_column('B:C', 30);
$worksheet->set_column('C:D', 25);
$worksheet->set_column('D:E', 15);

$format1 =& $workbook->addformat();
$format1->set_align('left');
$format1->set_size(11);
$format1->set_font("tahoma");

$columns = 4;
$i=0;
while($row = mysql_fetch_array($result)){
	for($j=0; $j<$columns; $j++){
		
			$worksheet->write($i+1, $j, $row[$j], $format1);
		}
	$i++;
}


$workbook->close();

header("Pragma: public");
header("Expires: 0");
header("Cache-Control: must-revalidate, post-check=0, pre-check=0");
header("Content-Type: application/force-download");
header("Content-Type: application/octet-stream");
header("Content-Type: application/download");
header("Content-Disposition: attachment; filename=".basename("$table.xls").";");
header("Content-Transfer-Encoding: binary ");
header("Content-Length: ".filesize($fname));
readfile($fname);
unlink($fname);
exit();
?>