Method each

examples/demo.php:5163–5169

* @param callable(TValue, TKey): void $callback * @return static */

(callable $callback)

     * @return static<TKey, TValue>
     */
    public function each(callable $callback)
    {
        foreach ($this->items as $key => $value) {
            $callback($value, $key);
        }
        return $this;
    }

    /** @return TValue|null */
    public function first(): mixed