I wonder how to get the success or failure message upon execution of a ffmpeg command

I am using a ffmpeg command for watermarking a video.
it does the work but i need to detect wheather it is executed successfully or not.
my command:

    $mark = "ffmpeg -i ".$inputvideo." -i logo.png -filter_complex ". '"overlay=x=(main_w-overlay_w):y=(main_h-overlay_h)"'." ".uniqid()."html56.mp4";

For output i used something like:

$x = exec($mark);
print_r($x);

But i am not getting anything printed in place of $x.
After some searching I found this statement for exec command

string exec ( string $command [, array &$output [, int &$return_var ]] )
$s=exec($mark,$var);

$var is my return var.

Now when i print $var i am getting an empty array.
Please suggest where i am missing.