$title = "название видео";
header('Content-Disposition: attachment; filename="'.$title.'.flv"');
$link = 'ссылка на flv';
$header = get_headers($link, 0);
$fp = @fopen($link, "rb");
if(isset($header)) foreach($header as $v) header($v);
while(!feof($fp)) echo fread($fp, BLOCK);
fclose($fp);