* @param string|null $current The current prefix
* @param string $sep The key separator
* @param string $prefix The key prefix
* @param string $suffix The key suffix
* @return array The flattened data
*/
* @param string|null $current The current prefix
* @param string $sep The key separator
* @param string $prefix The key prefix
* @param string $suffix The key suffix
* @return array The flattened data
*/