As I mentioned above, I was wondering if SIGKILL is defined on windows. I just had confirmation that it `SIGKILL` is not defined on windows environment.

We could define a Process constant : 

```php
class Process
{
    const SIGKILL = 9;

    public function stop($timeout = 10, $signal = self::SIGKILL)
    {
        // ...
    }
}
```

Another workaround is to define `SIGKILL` is the environment : 

```php
if (!defined('SIGKILL')) {
    define('SIGKILL', 9);
}

class Process
{
    public function stop($timeout = 10, $signal = SIGKILL)
    {
        // ...
    }
}
```

But I don't like both of these workaround so much