Watermark an animated gif

<?php
//This will save the watermarked image
$animation = "original.gif";
$watermark = "watermark.png";
$cmd = "$animation -coalesce -gravity Center -geometry +0+0 
null: $watermark -layers composite -layers optimize";
exec(convert $cmd output.gif);

// This will watermak on the fly
// The code is saved on a page of its own and called using the image tags with its page name.
$animation = "original.gif";
$watermark = "watermark.png";
$cmd = "convert $animation -coalesce -gravity Center -geometry +0+0 
null: $watermark -layers composite -layers optimize GIF:-";
header("Content-type: image/gif");
passthru($cmd, $retval);
?>