I've scripted this code to have the email sent both in html text and plain text. Though the email arrives and the from / subject fields are filled in, the message itself remains empty. I also can't switch from html text to plain text.

Where lies the error?

PHP Code:
<?php
// configure boundary string
$semi_rand md5(time()); 
$mime_boundary "==Multipart_Boundary_x{$semi_rand}x";

// initialize variables 
$to 'info@nomessageinmail.nl';
$subject 'Een vraag';
$from stripslashes(utf8_decode($_POST['from']));
$email stripslashes(utf8_decode($_POST['email']));
$comments stripslashes(utf8_decode($_POST['comments']));

// start multipart message
$message "This is a MIME encoded message."

//This is the text/plain version
$message .= "\r\n\r\n--" $mime_boundary "\r\n";
$message .= "Content-type: text/plain;charset=iso-8859-1\r\n\r\n";
$message .= 'Van: ' $from "\r\n";
$message .= 'Email: '$email ."\r\n";
$message .= 'Bericht: '$comments;

//This is the text/html version
$message .= "\r\n\r\n--" $mime_boundary "\r\n";
$message .= "Content-type: text/html;charset=iso-8859-1\r\n\r\n";
$message .= '<b>Van:</b> ' $from "\r\n";
$message .= 'Email: '$email ."\r\n";
$message .= 'Bericht: '$comments;
$message nl2br($message);

$message .= "\r\n\r\n--" $mime_boundary "--";

// add additional email headers for more user-friendly reply
$headers "MIME-Version: 1.0\r\n";
$headers .= "Content-Type: multipart/alternative;boundary=" $mime_boundary  "\r\n";
$headers .= "From: " mb_encode_mimeheader($from"iso-8859-1""Q") ." <" $email ">\r\n"
$headers .= "Reply-To: $email\r\n";

// send email message
$OK mail($to$subject$message$headers);
// let Flash know what the result was
if ($OK) {
  echo 
'sent=OK';
  }
  else {
  echo 
'sent=failed&reason='urlencode('Er is een probleem met de server. Probeer het later nog eens.');
  }
?>