* Returns the parameter value filtered as a boolean. Uses flag:
     * FILTER_VALIDATE_BOOLEAN
     */
    public function getBoolean(string $key, $default = false): bool
    {
        return $this->filter($key, $default, FILTER_VALIDATE_BOOLEAN);
    }

    /**
     * Returns the parameter value validated as a 2 character country code. If
     * the preg_match for exactly two alphabetic characters fails, the default
     * value is returned.
     */
    public function getCountryCode(string $key, string $default = ''): string
    {
        $country = strtoupper(trim($this->get($key, $default)));
        if (preg_match('/^[a-z]{2}$/i', $country)) {
            return $country;
        }

        return $default;
    }

    /**
     * Returns a boolean if the value is considered not empty.
     * @param array<TKey>|int|string|null $keys
     */
    public function isNotEmpty($keys = null): bool
    {
        return $this->isEmpty($keys) === false;
    }

    /**
     * Returns a boolean if the value of one of the keys is considered empty.
     * @param array<TKey>|int|string|null $keys
     */
    public function isOneEmpty($keys = []): bool
    {
        foreach ($keys as $key) {
            if ($this->isEmpty($key)) {
                return true;
            }
        }

        return false;
    }

    /**
     * Filter key.
     * @return mixed
     * @see http://php.net/manual/en/function.filter-var.php
     */
    public function filter(string $key, $default = null, int $filter = FILTER_DEFAULT, $options = [])
    {
        $value = $this->get($key, $default);
        // Always turn $options into an array - this allows filter_var option shortcuts.
        if (!\is_array($options) && $options) {
            $options = ['flags' => $options];
        }
        // Add a convenience check for arrays.
        if (\is_array($value) && !isset($options['flags'])) {
            $options['flags'] = FILTER_REQUIRE_ARRAY;
        }

        return filter_var($value, $filter, $options);
    }
}