code: public function filter($data = null, $filter = false)
      {
          if (is_null($data)) {
              return null;
          }
  
          if (is_array($data)) {
              return array_map(function ($value) use ($filter) {
                  return $this->filter($value, $filter);
              }, $data);
          }
  
          return ($filter == true ? $this->xssClean($data) : trim($data));
      }
docs: Filter method for HTTP Values.
  
  @param string $data
  @param bool   $filter
  
  @return string|null