phpmailer 内容乱码

<?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 = PHPMailer::ENCRYPTION_STARTTLS;
    $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 = '这里是邮件主题';
    $mail->Body    = '<h1>这里是邮件内容</h1>';
    $mail->Body    = mb_convert_encoding($mail->Body, 'HTML-ENTITIES', 'UTF-8');

    // 设置字符集
    $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}";
}
?>