php5 Header Error For Downloading

i am trying to force download with this but it give me error i don't know why

<?php

$url='http://www.zshare.net/video/'.$_GET['video'].'/';
$linkdown=$url;


// Headers for an download:
header('Content-Type: application/octet-stream');
header('Content-Disposition: attachment; filename="$linkdown"'); 
header('Content-Transfer-Encoding: binary');

?> 

PHP:

 

Is there a space or newline before your "<?php" tag? I say this because the first error is on line 9, but your first header command is on line 8, therefore I think your script has been moved down a line by a newline character. This means that a blank line is being outputted to the browser; then you have tried to send the headers. If there is one, please try removing it.

Jay