public function add(string $key)
    {
        foreach ($this->items as  &$items) {
            array_push($items, $key);
        }

        return $this;
    }
public function add(string $key)
    {
        foreach ($this->items as  &$items) {
            array_push($items, $key);
        }

        return $this;
    }
public function add ( string $ key ) { foreach ( $ this -> items as & $ items ) { array_push ( $ items , $ key ) ; } return $ this ; }
Add cached key.

@param string $key 缓存的key

@return static
Add cached key .