private function getEnvironmentVariables()
    {
        // Initialize from globals, allow override of special keys via putenv calls.
        $variables = $this->inheritEnvironment
            ? array_replace($_ENV, $_SERVER, $this->environment)
            : $this->environment;

        foreach (array_keys($variables) as $name) {
            if (false !== ($value = getenv($name))) {
                $variables[$name] = $value;
            }
        }

        return $variables;
    }