Calling FFmpeg through PHP proc_open always causes output to be sent STDERR instead of STDOUT

Question

I'm trying to call FFmpeg using proc_open and then I try to read the output of STDOUT but output is only ever sent to STDERR even when there is no error.

My code seems to be correct because when I use proc_open to call "ls -la" it sends the output to STDOUT. But I'm completely new to working with streams so I'm probably missing something.

I'd like to know if anyone knows what is happening or if I'm doing something wrong?

Here are three commands for test purposes:

$cmd = ['ls', '-la']; // sends to STDOUT
$cmd = ['ffmpeg', '-h']; // sends part of output to STDERR and some of it to STDOUT
$cmd = ['ffmpeg', '-i','in.mp4','-c:v','copy','out.mp4']; // sends all to STDERR

And here is my code:

// $cmd = ['ls', '-la'];
$cmd = ['ffmpeg', '-h'];

$descriptor_spec = [
    0 => ['pipe', 'r'],
    1 => ['pipe', 'w'],
    2 => ['pipe', 'w'],
];

$proc = proc_open($cmd, $descriptor_spec, $pipes, dirname(__FILE__), null);

$stdout = stream_get_contents($pipes[1]);
fclose($pipes[1]);
echo PHP_EOL . "STDOUT: {$stdout}" . PHP_EOL; 

$stderr = stream_get_contents($pipes[2]);
fclose($pipes[2]);
echo PHP_EOL . "STDERR: {$stderr}" . PHP_EOL; 

$exitCode = proc_close($proc);
var_dump($exitCode);

Answer 1

Found the reason for this behavior in another answer https://stackoverflow.com/a/35215447/5618007.

FFmpeg purposely outputs everything to stderr to keep stdout clear.