public static function escape_filter_value($values = array())
    {
        // Parameter validation
        if (!is_array($values)) {
            $values = array($values);
        }

        foreach ($values as $key => $val) {
            // Escaping of filter meta characters
            $val = str_replace('\\', '\5c', $val);
            $val = str_replace('*',  '\2a', $val);
            $val = str_replace('(',  '\28', $val);
            $val = str_replace(')',  '\29', $val);

            // ASCII < 32 escaping
            $val = self::asc2hex32($val);

            if (null === $val) $val = '\0';  // apply escaped "null" if string is empty

            $values[$key] = $val;
        }

        return $values;
    }