public function transform($config)
    {
        array_walk($config, function(&$value, $key) {
            if (in_array($key, $this->getKeys(), true)) {
                $value = $this->transformValue($value);
            } else {
                if ($this->shouldRecurse($value, $key)) {
                    $value = $this->transform($value);
                }
            }
        });

        return $config;
    }