private static function validateFilters($filters, $settings)
    {
        if (!is_array($filters)) {
            $filters = [$filters];
        }

        // Each fitler.
        foreach ($filters as $index => &$filter) {

            // Check this item.
            $filter = self::validateFilterItem($filter, $settings);

            // Remove if invalid.
            if (empty($filter)) {
                unset($filters[$index]);
            }
        }

        return $filters;
    }