Braintree\Util::_flattenUserKeys PHP Method

    private static function _flattenUserKeys($keys, $namespace = null)
    {
        $flattenedArray = [];
        foreach ($keys as $key => $value) {
            $fullKey = empty($namespace) ? $key : $namespace;
            if (!is_numeric($key) && $namespace != null) {
                $fullKey .= '[' . $key . ']';
            }
            if (is_numeric($key) && is_string($value)) {
                $fullKey .= '[' . $value . ']';
            }
            if (is_array($value)) {
                $more = self::_flattenUserKeys($value, $fullKey);
                $flattenedArray = array_merge($flattenedArray, $more);
            } else {
                $flattenedArray[] = $fullKey;
            }
        }
        sort($flattenedArray);
        return $flattenedArray;
    }