how to send a variable in a mail () of several parts?

Good morning, The problem I have is with a code to send emails with attachments that I found on the web and I want to modify it to send a variable, however when placing the variable it does not arrive in the mail, and verify that the variable is not empty so I guess it will be my mistake or something I do not know yet.

    $id=$_GET['id'];

        //The form has been submitted, prep a nice thank you message
    $output = '<h1>!</h1>';
    //Set the form flag to no display (cheap way!)
    $flags = 'style="display:none;"';

    //Deal with the email
    $to = '[email protected]';
    $subject = 'Subasta slip';

    $message = strip_tags($_POST['message']);
    $attachment = 
    chunk_split(base64_encode(file_get_contents($_FILES['file']['tmp_name'])));
    $filename = $_FILES['file']['name'];

    $boundary =md5(date('r', time())); 

    $headers = "From: [email protected]\r\nReply-To: [email protected]";
    $headers .= "\r\nMIME-Version: 1.0\r\nContent-Type: multipart/mixed; boundary=\"_1_$boundary\"";

    $message="This is a multi-part message in MIME format.

 --_1_$boundary
 Content-Type: multipart/alternative; boundary=\"_2_$boundary\"

 --_2_$boundary
 Content-Type: text/html; charset=\"iso-8859-1\"
 Content-Transfer-Encoding: 7bit
 //********************aquí es donde quiero enviar la variable****************
<h2>ID del producto</h2>
<p>".$id."</p> 

$message

--_2_$boundary--
--_1_$boundary
Content-Type: application/octet-stream; name=\"$filename\" 
Content-Transfer-Encoding: base64 
Content-Disposition: attachment 

$attachment
--_1_$boundary--";

    mail($to, $subject, $message, $headers);

As a side note, I can use any way to send the variable, I just need to see it somewhere. Thanks in advance