Bu şekilde dener misin?
exec('wget http://bilmemne.com/video/'.$video.'.mp4');
Veya
exec("wget http://bilmemne.com/video/{$video}.mp4");
exec('wget http://bilmemne.com/video/'.$video.'.mp4');
exec("wget http://bilmemne.com/video/{$video}.mp4");