Hello,
I'm using PHP 7.1 on IIS 7.5 and symfony/process always returns an empty output.

Here is a test case:

<?php
use Symfony\Component\Process\Process;

require_once __DIR__.'/vendor/autoload.php';

//This returns an output
var_dump(shell_exec('dir'));

$process = new Process('dir');
$process->mustRun();
//These return empty strings
var_dump(
    $process->getOutput(),
    $process->getErrorOutput()
);

What's strange is that it works when calling PHP (same binary) from the commandline.
And of course the same code works fine on my Linux/Apache server.

I tested various other commands (cd, Python.exe) and I always get the same empty result.

mustRun() does throw an exception if the command does not exist:

PHP Fatal error:  Uncaught Symfony\Component\Process\Exception\ProcessFailedException: The command "foobar" failed.

Exit Code: 1(General error)

Working directory: C:\inetpub\wwwroot

Output:
================


Error Output:
================
 in C:\inetpub\wwwroot\vendor\symfony\process\Process.php:241
Stack trace:
#0 C:\inetpub\wwwroot\test.php(11): Symfony\Component\Process\Process->mustRun()
#1 {main}
  thrown in C:\inetpub\wwwroot\vendor\symfony\process\Process.php on line 241

Edit: I also tried configuring IIS to use another PHP binary (installed with Chocolatey) and I get the same issue, so I guess it is linked to IIS itself.