phpThumb - Compressing GIF is loosing its' animation

Question

I've been trying to use phpThumb to compress images including GIF images:

                $phpThumb->resetObject();
                $phpThumb->setSourceData(file_get_contents('mydir/'.date('Y',$up_ts).'/'.date('m',$up_ts).'/'.$up_old_name));
                $output_filename = 'mydir/'.date('Y',$up_ts).'/'.date('m',$up_ts).'/'.$up_new_name;
                $phpThumb->setParameter('q', $img_quality);
                if($phpThumb->GenerateThumbnail()) {
                    if($phpThumb->RenderToFile($output_filename)) {
                    } else {
                    }
                    $phpThumb->purgeTempFiles();
                } else {
                }

ImageMagick is enabled on my server so phpThumb should be using it for compressing GIF images while keeping animation. However, this is not the case.

Any idea?

Many thanks, Zeid