You cannot do this during upload, however, after upload you can create a thumbnail.
You'll need to run this on the command line:
ffmpeg -itsoffset -4 -i uploaded_movie.avi -vcodec mjpeg -vframes 1 -an -f rawvideo -s 320x240 thumbnail.jpg
From PHP, after uploading and saving the file:
$output_file_name = '/var/www/your_site/images/thumbnail.jpg';
$cmd = 'ffmpeg -itsoffset -4 -i ' . $path_to_uploaded_file. ' -vcodec mjpeg -vframes 1 -an -f rawvideo -s 320x240 ' . $output_file_name;