It tells the client to close the connection. It works fine on httpd && chrome.
ob_start();
echo "Backgrounding...";
// get the size of the output
$size = ob_get_length();
// send headers to tell the browser to close the connection
header("Content-Length: $size");
header('Connection: close');
// close current session
if (session_id()) session_write_close();
// flush all output
ob_end_flush();
ob_flush();
flush();