php mailform中的链接选择菜单

如何解决php mailform中的链接选择菜单

只需通过Xin Yang安装链接的选择菜单。 当我尝试将所选内容放入邮件表单中时，这是我所做的：

<select class="custom-select" type="text" name="firstlevel" id="firstlevel" required></select>
<select class="custom-select" type="text" name="secondlevel" id="secondlevel" required></select>
<select class="custom-select" type="text" name="thirdlevel" id="thirdlevel" required></select>

邮件形式的php：

<?php

$ip = $_SERVER['REMOTE_ADDR']; // get ip to short variable name for logging purposes 

$to = "[e-mail]";
$subject= "request via website";
$name = $_POST['name'];
$firstlevel = $_POST['firstlevel'];
$secondlevel = $_POST['secondlevel'];
$thirdlevel = $_POST['thirdlevel'];

$remarks = $_POST['remarks'];

$message = "
Name: $name
Requested workshop:
$firstlevel
$secondlevel
$thirdlevel

Remarks: $remarks

";
  $mime_boundary="==Multipart_Boundary_x".md5(mt_rand())."x";
     $headers = "From: $email\r\n" .
     "MIME-Version: 1.0\r\n" .
        "Content-Type: multipart/mixed;\r\n" .
        " boundary=\"{$mime_boundary}\"";
     $message = "This is a multi-part message in MIME format.\n\n" .
        "--{$mime_boundary}\n" .
        "Content-Type: text/plain; charset=\"iso-8859-1\"\n" .
        "Content-Transfer-Encoding: 7bit\n\n" .
     $message . "\n\n";
     foreach($_FILES as $userfile)
     {
        $tmp_name = $userfile['tmp_name'];
        $type = $userfile['type'];
        $name = $userfile['name'];
        $size = $userfile['size'];
        if (file_exists($tmp_name))
        {
           if(is_uploaded_file($tmp_name))
           {
              $file = fopen($tmp_name,'rb');
              $data = fread($file,filesize($tmp_name));
              fclose($file);
              $data = chunk_split(base64_encode($data));
           }
           $message .= "--{$mime_boundary}\n" .
              "Content-Type: {$type};\n" .
              " name=\"{$name}\"\n" .
              "Content-Disposition: attachment;\n" .
              " filename=\"{$fileatt_name}\"\n" .
              "Content-Transfer-Encoding: base64\n\n" .
           $data . "\n\n";
        }
     }
     $message.="--{$mime_boundary}--\n";
if (mail($to,$subject,$message,$headers))
   echo "<b>Thanks for your request.</b><br>It will be processed very shortly,so I will get back to you soon.<br>Please stay healthy in the meantime.
<br>&nbsp;<br>&nbsp;<br><blockquote class='bordered layout-2'><b>This was your request:</b>
<br>&nbsp;<br>$name
<br>&nbsp;<br>$firstlevel,$secondlevel,$thirdlevel
<br>&nbsp;<br>Remarks:<br>$remarks</blockquote>
";
else
   echo "Your request has not been processed.<br>Please try again,filling in all fields.";
?>

但是似乎$ firstlevel，$ secondlevel和$ thirdlevel无法识别为值，因为它们既不在发送的邮件中，也不在发送的邮件的摘要中。 请如何使这些价值观发挥作用？ 谢谢。