Download file program using php on unix box

I have created a beginner program to forcefully download file from unix box to windows through browser, it is not throwing any error but shows nothing on browser just a blank page.

PHP version- 5.2.13
Apache-2.0
Unix Box- HP-UX 11.11 (old version latest is 11.31)
local PC- windows XP Prof.
Browser- IE 7, Mozilla.

Below is my code (this code resides on unix box):

<?php
    ob_start();
    $file = '/opt/hpws/apache/htdocs/barn/file2';

    if (file_exists($file)) {
        header('Content-Description: File Transfer');
        header('Content-Type: application/octet-stream;charset=utf-8');
        header('Content-Disposition: attachment; filename=$file');
        header('Content-Transfer-Encoding: binary');
        header('Expires: 0');
        header('Cache-Control: must-revalidate');
        header('Pragma: public');
        header('Content-Length: ' . filesize($file));

        readfile($file);
        exit;
    }
?>

Copyright Notice：Content Author:「theartist33」，Reproduced under the CC 4.0 BY-SA copyright license with a link to the original source and this disclaimer.
Link to original article：https://stackoverflow.com/questions/18677191/download-file-program-using-php-on-unix-box