Method groupBy

examples/demo.php:5145–5147

* @template TGroupKey of array-key * * @param (callable(TValue, TKey): TGroupKey)|array|string $groupBy * @param bool $preserveKeys * @return static< * ($groupBy is (array|string) * ? array-key * : TGroupKey), * static<($preserveKeys is true ? TKey : int), TValue> * > */

($groupBy, $preserveKeys = false)

     * >
     */
    public function groupBy($groupBy, $preserveKeys = false)
    {
    }

    /**
     * @template TMapValue