code: public function addFilterItem($item)
      {
          if (!($item instanceof self)) {
              if (is_array($item)) {
                  try {
                      $item = new self($item);
                  } catch (\Exception $e) {
                      trigger_error('Could not auto-instantiate FilterItem. ' . $e->getMessage(), E_USER_WARNING);
                  }
              } else {
                  trigger_error('Array parameter item is not of expected type "FilterItem"!', E_USER_WARNING);
              }
          }
          $this->filterItems[] = $item;
  
          return $this;
      }
docs: Adds an object of "FilterItems" of the FilterItem.
  
  @param FilterItem|array $item
  
  @return FilterItem