// Save Featured Image If It's a youtube video
add_action('save_post', 'wds_video_sideload_post_thumb');
function wds_video_sideload_post_thumb() {
global $post;

    $youtube_url = get_post_meta( $post->ID, 'videobox', true );
    $youtubeid = youtubeid($youtube_url);
        $thumb_url = 'http://img.youtube.com/vi/'. $youtubeid .'/0.jpg';

        require_once(ABSPATH . 'wp-admin/includes/file.php');
        require_once(ABSPATH . 'wp-admin/includes/media.php');
        set_time_limit(300);

        if ( ! empty($thumb_url) ) {
            // Download file to temp location
            $tmp = download_url( $thumb_url );

            // Set variables for storage
            // fix file filename for query strings
            preg_match('/[^\?]+\.(jpg|JPG|jpe|JPE|jpeg|JPEG|gif|GIF|png|PNG)/', $thumb_url, $matches);
            $file_array['name'] = basename($matches[0]);
            $file_array['tmp_name'] = $tmp;

            // If error storing temporarily, unlink
            if ( is_wp_error( $tmp ) ) {
                @unlink($file_array['tmp_name']);
                $file_array['tmp_name'] = '';
            }

            // do the validation and storage stuff
            $thumbid = media_handle_sideload( $file_array, $post->ID, $desc );
            // If error storing permanently, unlink
            if ( is_wp_error($thumbid) ) {
                @unlink($file_array['tmp_name']);
                return $thumbid;
            }
        }

        set_post_thumbnail( $post, $thumbid );
}

function youtubeid($url) {
        if (preg_match('%youtube\\.com/(.+)%', $url, $match)) {
                $match = $match[1];
                $replace = array("watch?v=", "v/", "vi/");
                $match = str_replace($replace, "", $match);
                $match = substr( $match, 0, ( strpos( $match, '&')  ) );
        }
        return $match;
}