ติดปัญหา Warning: mb_strlen() [function.mb-strlen]: Unknown encoding "tis-620" in phpmailer\class.phpmailer.php on line 1585 และ on line 1604

<?PHP
$mail = new PHPMailer();

//$body = "ทดสอบการส่งอีเมล์ภาษาไทย UTF-8 ผ่าน SMTP Server ด้วย PHPMailer.";
$body = $detail;
$body             = preg_replace('/\\\\/','', $body); //Strip backslashes
$subject             = preg_replace('/\\\\/','', $subject); //Strip backslashes

$mail->CharSet = "tis-620";
$mail->IsSMTP();
$mail->SMTPDebug = 0;
$mail->SMTPAuth = true;
$mail->Host = "mail.xxxxxxx.com"; // SMTP server
$mail->Port = 25; // พอร์ท
$mail->Username = "[email protected]"; // account SMTP
$mail->Password = "xxxxx"; // รหัสผ่าน SMTP

$mail->SetFrom("[email protected]", "xx Office Staff");
$mail->AddReplyTo("[email protected]", "xx Office Staff");
$mail->Subject = $subject;

$mail->MsgHTML($body);

$mail->AddAddress("[email protected]", "recipient1"); // ผู้รับคนที่หนึ่ง
//$mail->AddAddress("[email protected]", "recipient2"); // ผู้รับคนที่สอง

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