Shell_exec('net user ....) change password windows users

I wanted to change a user’s login password to windows
with
shell_exec(‘net user user002 pwd123456 2>&1’);
written like this it works

but if i insert a variable it doesn’t work
$usr = ‘user002’;
shell_exec(‘net user ’ . $usr . ’ pwd123456 2>&1’);

Someone can help me
thank you