public static function flatten(array $array, string $separator = null, string $prepend = ''): array
    {
        $flattened = [];

        foreach ($array as $key => $value) {
            if (is_array($value)) {
                $flattened = array_merge($flattened, static::flatten($value, $separator, $prepend . $key . $separator));
            } else {
                $flattened[$prepend . $key] = $value;
            }
        }

        return $flattened;
    }