I was quite stuck when trying to make my script redirect the client to another URL and then continue processing. The reason was php-fpm. All possible buffer flushes did not work, unless I called fastcgi_finish_request();
For example:
<?php
// redirecting...
ignore_user_abort(true);
header("Location: ".$redirectUrl, true);
header("Connection: close", true);
header("Content-Length: 0", true);
ob_end_flush();
flush();
fastcgi_finish_request(); // important when using php-fpm!
sleep (5); // User won't feel this sleep because he'll already be away
// do some work after user has been redirected
?>