this is not the same as
http://bugs.php.net/bug.php?id=19711 :)

the following code on my platform displays:
File size: 0, written: 2890
File size: 2890, written: 2890

as far as i know fflush should flush output to file at the moment of calling :] moving the line with flcose to the end of script causes that also the second display shows
File size: 0, written: 2890
it looks for me that fflush doesn`t work and all flushing is done during file closing.
tested it under linux also - both variants are done ok there:
File size: 2890, written: 2890
File size: 2890, written: 2890 

$str = '';
for($i = 0;$i < 1000;$i++) {
	$str .= $i;
}

$fp = fopen('test.txt', 'wb');
$written = fwrite($fp, $str);
fflush($fp);
clearstatcache();
$size = filesize('test.txt');
echo 'File size: '.$size.', written: '.$written.'<br>';
fclose($fp);
clearstatcache();
$size = filesize('test.txt');
echo 'File size: '.$size.', written: '.$written.'<br>';