How to add error_log($shell)?

[chrisj]

How do I add error_log($shell) to determine the $shell output from the command to see what its value is from this code:

    if ($video_res >= 854 || $video_res == 0) {
        $shell     = shell_exec("$ffmpeg_b -y -i $video_file_full_path -i /home/user/public_html/watermark.jpg -filter_complex 'overlay=x=(main_w-overlay_w)/2:y=(main_h-overlay_h)/2' -vcodec libx264 -preset {$pt->config->convert_speed} -filter:v scale=854:-2 -crf 26 $video_output_full_path_480 2>&1");
        $upload_s3 = PT_UploadToS3($filepath . "_480p_converted.mp4");
        $db->where('id', $insert);
        $db->update(T_VIDEOS, array(
            '480p' => 1
        ));
    }