public function mapKeys(callable $callback)
    {
        $items = [];

        foreach ($this->items as $key => $value) {
            $items[$callback($key, $value)] = $value;
        }

        return $this->createFrom($items);
    }
public function mapKeys(callable $callback)
    {
        $items = [];

        foreach ($this->items as $key => $value) {
            $items[$callback($key, $value)] = $value;
        }

        return $this->createFrom($items);
    }
public function mapKeys ( callable $ callback ) { $ items = [ ] ; foreach ( $ this -> items as $ key => $ value ) { $ items [ $ callback ( $ key , $ value ) ] = $ value ; } return $ this -> createFrom ( $ items ) ; }
Applies the given function to each _key_ in the bag and returns
a new bag with the keys returned by the function and their values.

@param callable $callback Function is passed `($key, $value)`

@return static
Applies the given function to each _key_ in the bag and returns a new bag with the keys returned by the function and their values .