Can't execute exec command (php / windows / ffmpeg)

Question

I'm trying to run this command that does work in cli

ffmpeg -i tmp1.flv -c:a copy -vf drawbox=:x=0:y=0:color=invert:t=2 output1.flv

I was able to run only ffmpeg -version by php.

$res = exec("C:\\wamp\\www\\ffmpeg\\bin\\ffmpeg.exe ffmpeg -version");

Location of tmp1.flv file:

C:\wamp\www\tmp1.flv

When I'm running $res = exec("chdir") => C:\\wamp\\www\\

How can I run this by php

ffmpeg -i tmp1.flv -c:a copy -vf drawbox=:x=0:y=0:color=invert:t=2 output1.flv

I tried:

$res = exec("C:\\wamp\\www\\ffmpeg\\bin\\ffmpeg.exe ffmpeg -i tmp1.flv -c:a copy -vf drawbox=:x=0:y=0:color=invert:t=2 output1.flv", $output, $return_var);
$res = exec("C:\\wamp\\www\\ffmpeg\\bin\\ffmpeg.exe ffmpeg -i C:\\wamp\\www\\tmp1.flv -c:a copy -vf drawbox=:x=0:y=0:color=invert:t=2 C:\\wamp\\www\\output1.flv", $output, $return_var);
$res = exec("C:\\wamp\\www\\ffmpeg\\bin\\ffmpeg.exe ffmpeg -i C:\wamp\www\tmp1.flv -c:a copy -vf drawbox=:x=0:y=0:color=invert:t=2 C:\wamp\www\output1.flv", $output, $return_var);
$res = exec("C:\\wamp\\www\\ffmpeg\\bin\\ffmpeg.exe ffmpeg -i \"C:\wamp\www\tmp1.flv\" -c:a copy -vf drawbox=:x=0:y=0:color=invert:t=2 \"C:\wamp\www\output1.flv\"", $output, $return_var);
$res = exec("C:\\wamp\\www\\ffmpeg\\bin\\ffmpeg.exe ffmpeg -i 'C:\wamp\www\tmp1.flv' -c:a copy -vf drawbox=:x=0:y=0:color=invert:t=2 'C:\wamp\www\output1.flv'", $output, $return_var);
...

but echo $res and echo $output gives nothing, and echo $return_var gives 1

+++UPDATE+++

Made a little progress thanks to this post , it does give me output and info about the file, but still can't run the whole command.

$cmd = 'C:\\wamp\\www\\ffmpeg\\bin\\ffmpeg.exe ffmpeg -i tmp1.flv 2>&1';

exec($cmd, $output, $value);

var_dump($output);
var_dump($value);
var_dump($cmd)

+++UPDATE 2+++

echo exec('whoami') => nt authority\\system

Answer 1

I had ffmpeg after ffmpeg.exe - that's why it didn't work.

$cmd = 'C:\\wamp\\www\\ffmpeg\\bin\\ffmpeg.exe -i tmp1.flv -c:a copy  -vf drawbox=:x=0:y=0:color=invert:t=2 output2.flv 2>&1';
exec($cmd, $output)