How can I start a WSL-based Linux executable from within PHP on Windows and capture its stdout?

I have a program that only works on Linux, and I have my development environment on Windows.

When I run a WSL command from cmd in Windows it works as expected.
- When I run C:\wsl echo "foo" in cmd.exe I get foo back via stdout.
When I run the same command from within php via exec I don't get any stdout output and get an exit code -1073740791 which I assume is an error.

Cmd.exe:

C:\wsl echo "foo"
foo

PHP:

<?php
    $cmd = 'wsl echo "foo"';
    exec($cmd, $out, $code);
    dd($out, $code);
?>
// $out is []
// $code is -1073740791

Solution

As discussed in the comments, the underlying issue was that your systemwide Apache web-server service (wampapache64) was configured to run as NT Authority\SYSTEM (aka Local System or LocalSystem).

...and running anything like a web-server as LocalSystem is a very, very bad idea for other reasons, especially when it's running PHP code - a simple mistake in uploaded-file handling or a failure to correctly sanitize user input could potentially hose your entire computer, especially when it's a publicly-exposed web-server that a malicious attacker can connect to.

Anyway:

Apparently when a process is running as SYSTEM it cannot use Win32's CreateProcess to invoke wsl to then indirectly invoke a Linux program.

PHP's exec function is implemented like so:
- The exec function wraps PHP's internal VCWD_POPEN.
- VCWD_POPEN is a macro defined as #define VCWD_POPEN(command, type) virtual_popen(command, type)
- virtual_popen varies based on OS:
  - On Win32, virtual_popen simply invokes popen_ex(command, type, CWDG(cwd).cwd, NULL).
    - And popen_ex calls CreateProcessAsUserW or CreateProcessW to actually start the new process.
    - I note that popen_ex uses OpenThreadToken to ensure the new process is created with the same user-account and permissions as the parent process - in this case NT Authority\SYSTEM.
So PHP's exec() is not just a simple wrapper over C's system().
Whereas if exec() was a wrapper over system() then programs would be invoked via cmd.exe which will behave differently (though that too would likely fail as well)
And WSL does not support running under Local System.

So by changing your web-server to run as a real user account that meant that the CreateProcess... call invoked wsl within a supported environment.