PHP - codigo php no funciona

<?php

$to='mio@gmail.com';

$from_user='yo';

$from_email='otro@hotmail.com';

$subject = '';

$message = '';

function mail_utf8($to, $from_user, $from_email,

                                             $subject = '(No subject)', $message = '')

   {

      $from_user = "=?UTF-8?B?".base64_encode($from_user)."?=";

      $subject = "=?UTF-8?B?".base64_encode($subject)."?=";

      $headers = "From: $from_user <$from_email>\r\n".

               "MIME-Version: 1.0" . "\r\n" .

               "Content-type: text/html; charset=UTF-8" . "\r\n";

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

   }

?>

$from_user = "=?UTF-8?B?".base64_encode($from_user)."?=";