I’m having trouble with the passthru() function when redirecting stderr to stdout

Could you help me with this strange passthru() behavior?

This code writes the “Division by zero” error to test.log:

passthru('php -r "echo 1/0;" 2>logs/test.log >> logs/test.log');

But this code doesn’t. It writes the error to console:

passthru('php -r "echo 1/0;" 2>&1 >> logs/test.log');

I was expecting that the stderr is redirected also to stdout.
Is this a known issue with passthru?

I’m running this in a CLI script.