Must sepcify output file... FFMPEG ERROR

If youll look in the /include/classes/class.video_duration.php.. theres this lil tid-bit:

class video_duration {

function find_video_duration_ffmpeg($duration_arr) {
global $config;
$video = $duration_arr['src'];
$debug = isset($duration_arr['debug']) ? $duration_arr['debug'] : 0;
$cmd = $config['ffmpeg'] . " -i " . $video; //THIS NEEDS TO BE CHANGED..
@exec("$cmd 2>&1", $output);
$output_all = implode("\n",$output);

if ($debug) {
echo "

$cmd</p>";
echo "<pre>"; print_r($output_all);
echo "</pre>";
}
--

right noe this:
$cmd = $config['ffmpeg'] . " -i " . $video;

is NOT specifying the proper output file (the flv file name) so all we have to do is put in the flv file path and it will be sucesfull. I tried it manually. but i have NO idea what var to use for the output file. anyone care to finish this off?

$cmd = $config['ffmpeg'] . " -i " . $video;

should be something like... but idk what $path_to_flv_video is...

cmd = $config['ffmpeg'] . " -i " . $video . " " . $path_to_flv_video;