使用PHP发送多部分/备用电子邮件的问题

composer require phpmailer/phpmailer

<?php
use PHPMailer\PHPMailer\PHPMailer;
use PHPMailer\PHPMailer\Exception;

require 'vendor/autoload.php';

$mail = new PHPMailer(true);

try {
    // 邮件服务器设置
    $mail->SMTPDebug = 0;                                       
    $mail->isSMTP();                                            
    $mail->Host       = 'smtp.example.com';                     
    $mail->SMTPAuth   = true;                                   
    $mail->Username   = 'your_email@example.com';                     
    $mail->Password   = 'your_email_password';                              
    $mail->SMTPSecure = 'tls';         
    $mail->Port       = 587;

    // 发件人、收件人设置
    $mail->setFrom('your_email@example.com', 'Mailer');
    $mail->addAddress('recipient@example.com', 'Recipient');

    // 邮件内容设置
    $mail->isHTML(true);                                  
    $mail->Subject = 'Subject of the Email';
    $mail->Body    = 'This is the <b>HTML</b> part of the email.';
    $mail->AltBody = 'This is the plain text part of the email.';

    // 添加附件
    $mail->addAttachment('path/to/file.pdf', 'file.pdf');

    // 发送邮件
    $mail->send();
    echo 'Message has been sent';
} catch (Exception $e) {
    echo "Message could not be sent. Mailer Error: {$mail->ErrorInfo}";
}