在PHP中，我们可以使用mail()函数来发送邮件，以下是一个简单的示例：

<?php
$to = "收件人邮箱";
$subject = "邮件主题";
$message = "邮件内容";
$headers = "From: 发件人邮箱" . "rn" .
    "ReplyTo: 回复邮箱" . "rn" .
    "XMailer: PHP/" . phpversion();
if(mail($to, $subject, $message, $headers)) {
    echo "邮件发送成功";
} else {
    echo "邮件发送失败";
}
?>

在这个示例中，我们首先定义了收件人邮箱、邮件主题和邮件内容，我们创建了一个包含发件人邮箱、回复邮箱和邮件客户端信息的头部信息，我们使用mail()函数发送邮件，并根据返回值判断邮件是否发送成功。

问题与解答：

1、如何使用PHP发送带有附件的邮件？

答：要使用PHP发送带有附件的邮件，可以使用mail()函数的第三个参数传递一个包含附件路径的字符串。

<?php
$to = "收件人邮箱";
$subject = "邮件主题";
$message = "邮件内容";
$headers = "From: 发件人邮箱" . "rn" .
    "ReplyTo: 回复邮箱" . "rn" .
    "XMailer: PHP/" . phpversion();
// 添加附件
$file_path = "/path/to/your/attachment.txt";
$file_name = basename($file_path);
$encoded_string = chunk_split(base64_encode(file_get_contents($file_path)));
$mime_boundary = md5("===" . rand());
$headers .= "MIMEVersion: 1.0" . "rn";
$headers .= "ContentType: multipart/mixed; boundary="" . $mime_boundary . """ . "rn";
$body = "" . $mime_boundary . "rn";
$body .= "ContentType: text/plain; charset="UTF8"" . "rn";
$body .= "ContentTransferEncoding: base64" . "rn";
$body .= "rn" . chunk_split(base64_encode($message)) . "" . $mime_boundary . "" . "rn";
$body .= "ContentType: application/octetstream" . "rn";
$body .= "ContentDisposition: attachment; filename="" . $file_name . """ . "rn";
$body .= "ContentTransferEncoding: base64" . "rn";
$body .= "rn" . $encoded_string . "" . $mime_boundary . "" . "rn";
if(mail($to, $subject, $body, $headers)) {
    echo "邮件发送成功";
} else {
    echo "邮件发送失败";
}
?>

2、如何设置邮件的优先级？

答：要设置邮件的优先级，可以在mail()函数的第四个参数中添加XPriority头，要将优先级设置为高（1），可以这样做：

<?php
$to = "收件人邮箱";
$subject = "邮件主题";
$message = "邮件内容";
$headers = "From: 发件人邮箱" . "rn" .
    "ReplyTo: 回复邮箱" . "rn" .
    "XMailer: PHP/" . phpversion();
$headers .= "XPriority: 1" . "rn"; // 设置优先级为高
if(mail($to, $subject, $message, $headers)) {
    echo "邮件发送成功";
} else {
    echo "邮件发送失败";
}
?>