how to handle long process?

I am planning on a plug-in that will require a significant amount of processing after the user POST the form. I would like to be able to redirect the user to another page while the process is still running. Here’s what I had in mind, but the header redirection wouldn’t start until the whole script is completed. Any idea? I thought of using exec() to spawn another script, but there would pose some portability issue (unix vs. win32).

<?php
ignore_user_abort(TRUE);
header(“Location: http://something.com/somepage.html&#8221;);
ob_flush();
flush();

// the following is to simulate some long task that will happen…

$handle = fopen (dirname(__FILE__) . ‘/data.txt’, “w”);
if ($handle) {
for ($i = 0; $i < 10; $i++) {
// echo date(“Y-M-d G:i:s”) . ” Try number ” . $i . “\r\n”;
fwrite($handle, date(“Y-M-d G:i:s”) . ” Try number ” . $i . “<br>\r\n”);
sleep(2);
}
fwrite($handle, “Finally done!!!\r\n”);
}
fclose($handle);

?>