如何编写PHP双重加入订阅表格

<?php
// 连接到数据库
$dsn = 'mysql:host=localhost;dbname=mydatabase';
$username = 'myusername';
$password = 'mypassword';
$db = new PDO($dsn, $username, $password);

// 处理表单提交
if ($_SERVER['REQUEST_METHOD'] === 'POST') {
  // 验证电子邮件地址
  $email = filter_var($_POST['email'], FILTER_VALIDATE_EMAIL);
  if (!$email) {
    die('Invalid email address');
  }

  // 将用户信息存储到数据库
  $stmt = $db->prepare('INSERT INTO subscribers (email, date_subscribed) VALUES (:email, NOW())');
  $stmt->bindParam(':email', $email);
  $stmt->execute();

  // 发送确认电子邮件
  $to = $email;
  $subject = 'Confirm your subscription';
  $message = 'Thank you for subscribing to our newsletter. Please click the following link to confirm your subscription: https://example.com/confirm-subscription.php?email=' . urlencode($email);
  mail($to, $subject, $message);

  echo 'Thank you for subscribing!';
}

// 显示表单
?>

<form method="post">
 <label for="email">Email address:</label>
 <input type="email" id="email" name="email" required>
 <button type="submit">Subscribe</button>
</form>