code: public function sortKeys($order = SORT_ASC, $flags = SORT_REGULAR)
      {
          $this->validateSortArgs($order, $flags);
  
          $items = $this->items;
  
          if ($order === SORT_ASC) {
              ksort($items, $flags);
          } else {
              krsort($items, $flags);
          }
  
          return $this->createFrom($items);
      }
docs: Returns a bag with the keys sorted.
  
  Sorting flags:
  Constant                        | Description
  ------------------------------- | ------------------------
  `SORT_REGULAR`                  | compare values without changing types
  `SORT_NUMERIC`                  | compare values numerically
  `SORT_STRING`                   | compare values as strings
  `SORT_STRING | SORT_FLAG_CASE`  | compare values as strings ignoring case
  `SORT_LOCALE_STRING`            | compare values as strings based on the current locale
  `SORT_NATURAL`                  | compare values as strings using "natural ordering"
  `SORT_NATURAL | SORT_FLAG_CASE` | compare values as strings using "natural ordering" ignoring case
  
  @param int $order `SORT_ASC` or `SORT_DESC`
  @param int $flags Sorting flags to modify the behavior
  
  @return static