file download through php

I have a problem when doing indirect download of file through php, when
I click on download link Firefox and Internet Explorer give me same
options: Open and Save. Firefox opens file directly from Internet and
downloads without problem.

Internet Explorer saves file on local disk and I can open it from disk
correctly but when I try to open it directly from Internet it responds
with "There was an error opening this document. This file cannot be
found.", concrete situation in my usage is Pdf document. Below is my
concrete code, any suggestions are appreciated.

<?php
ob_start();
include_once("config.php");
ini_set('display_errors', 'Off');
error_reporting(0);

$args = trim($_SERVER["PATH_INFO"]);
$args = preg_replace("/^\//i", '', $args);
$argv = explode('/', $args);
$id = (int)$argv[0];

import('dao.fm.file');
$f = fm_file::instance($id);

if($f->get('id') < 1) {
        die('No file.');
}
header('Content-Disposition: attachment');
header('Accept-Charset: UTF-8');
header('Content-Type: '.$f->get('type').'; charset="UTF-8";');
readfile(get('FILES').'/'.$f->get('id'));
ob_end_flush();
?>
Received on Mon May 1 02:35:21 2006