function encode_subject($subject,$charset){
	$return = "=?".$charset."?Q?".str_replace("=\r\n", "", preg_replace("/\?/", "=3F", imap_8bit($subject)))."?=";
	return $return;
}

$subject = "Assunto é com acento";

mail($to, encode_subject($subject,"ISO-8859-1"), "", $headers)