如何使用 PHP 内置函数发送和接收电子邮件？

<?php
// 设置邮件参数
$to = 'recipient@example.com';
$subject = 'Test Email';
$message = 'Hello there! This is a test email.';
$headers = "From: sender@example.com\r\n";
$headers .= "Reply-To: sender@example.com\r\n";
$headers .= "MIME-Version: 1.0\r\n";
$headers .= "Content-Type: text/plain; charset=utf-8\r\n";

// 发送邮件
if (mail($to, $subject, $message, $headers)) {
  echo "Email sent successfully.";
} else {
  echo "Email could not be sent.";
}
?>

<?php
// 检查邮件
$mailbox = pop3_open('{pop.example.com:110}INBOX', 'username', 'password');

// 读取消息
$messages = pop3_get_all($mailbox);

// 输出消息
foreach ($messages as $message) {
  echo 'From: ' . $message['from'] . PHP_EOL;
  echo 'Subject: ' . $message['subject'] . PHP_EOL;
  echo 'Body: ' . $message['body'] . PHP_EOL;
  echo '-----------------------' . PHP_EOL;
}

// 关闭邮箱连接
pop3_close($mailbox);
?>

<?php
// 发送带有附件的电子邮件
$to = 'recipient@example.com';
$subject = 'Email with Attachment';
$message = 'Please find the attached document for your review.';
$attachment = 'document.pdf';
$headers = "From: sender@example.com\r\n";
$headers .= "Reply-To: sender@example.com\r\n";
$headers .= "MIME-Version: 1.0\r\n";
$headers .= "Content-Type: multipart/mixed; boundary=\"==BOUNDARY==\"\r\n";
$headers .= "Content-Transfer-Encoding: 7bit\r\n";

// 准备邮件正文
$body = "This is a MIME encoded message.\r\n\r\n";
$body .= "--==BOUNDARY==\r\n";
$body .= "Content-Type: text/plain; charset=\"UTF-8\"\r\n";
$body .= "Content-Transfer-Encoding: 7bit\r\n\r\n";
$body .= $message . "\r\n";

// 附加文件
$body .= "--==BOUNDARY==\r\n";
$body .= "Content-Type: application/octet-stream; name=\"" . basename($attachment) . "\"\r\n";
$body .= "Content-Disposition: attachment\r\n";
$body .= "Content-Transfer-Encoding: base64\r\n\r\n";
$body .= chunk_split(base64_encode(file_get_contents($attachment))) . "\r\n";

// 发送电子邮件
if (mail($to, $subject, $body, $headers)) {
  echo "Email with attachment sent successfully.";
} else {
  echo "Email could not be sent.";
}
?>