phpmailer 标题乱码

<?php
require 'vendor/autoload.php';

use PHPMailer\PHPMailer\PHPMailer;
use PHPMailer\PHPMailer\Exception;

$mail = new PHPMailer(true);

try {
    // 邮件服务器设置
    $mail->isSMTP();
    $mail->Host = 'smtp.example.com';
    $mail->SMTPAuth = true;
    $mail->Username = 'your_email@example.com';
    $mail->Password = 'your_password';
    $mail->SMTPSecure = 'tls';
    $mail->Port = 587;

    // 发件人
    $mail->setFrom('from@example.com', 'Mailer');

    // 收件人
    $mail->addAddress('to@example.com', 'Receiver');

    // 邮件内容
    $mail->isHTML(true);
    $mail->Subject = '邮件标题'.mb_convert_encoding('测试', 'UTF-8', 'GBK');
    $mail->Body    = '这是一封测试邮件。';

    // 设置邮件头字符集
    $mail->CharSet = 'UTF-8';

    $mail->send();
    echo '邮件发送成功';
} catch (Exception $e) {
    echo "邮件发送失败: {$mail->ErrorInfo}";
}
?>