PHP Mailer头编码的问题

<?php
require_once('../class.phpmailer.php');
//include("../class.smtp.php"); // optional,gets called from within class.phpmailer.php if not already loaded
iconv_set_encoding("internal_encoding","UTF-8");

$mail             = new PHPMailer();
$mail->CharSet = "utf8";
/*
$body             = file_get_contents('contents.html');
$body             = eregi_replace("[]",'',$body);
*/
$mail->IsSMTP(); // telling the class to use SMTP
//$mail->Host       = "mail.app-cpr.com"; // SMTP server
//$mail->SMTPDebug  = 2;                     // enables SMTP debug information (for testing)
                                           // 1 = errors and messages
                                           // 2 = messages only
$mail->SMTPAuth   = true;                  // enable SMTP authentication
$mail->SMTPSecure = "tls";                 // sets the prefix to the servier
$mail->Host       = "smtp.gmail.com";      // sets GMAIL as the SMTP server
$mail->Port       = 25;                   // set the SMTP port for the GMAIL server
$mail->Username   = "xxxxxxx@gmail.com";  // GMAIL username
$mail->Password   = "xxxxxxxxxxx";            // GMAIL password    
$mail->SetFrom('xxxxxxx@gmail.com','First Last');

$mail->AddReplyTo("xxxxxxx@gmail.com","First Last");

$mail->Subject    = "?????";
/*
$mail->AltBody    = "To view the message,please use an HTML compatible email viewer!"; // optional,comment out and test
*/
$mail->MsgHTML("?????");

$address = "xxxxxxx@gmail.com";
$mail->AddAddress($address,"John Doe");

if(!$mail->Send()) {
  echo "Mailer Error: " . $mail->ErrorInfo;
} else {
  echo "Message sent!";
}

?>

电子邮件发送正常,但标题有问题.

主题：’ทดสอบ’

我会尝试
– php mail special characters utf8
– use => $mail-> Subject =“=？UTF-8？B？”.base64_encode($subject).“？=”;

但它对我不起作用.我怎么知道标题编码是什么,并在我的泰语语言中解决这个问题？