public function containsKey($key): bool
    {
        return isset($this->items[$key]) || array_key_exists($key, $this->items);
    }
public function containsKey($key): bool
    {
        return isset($this->items[$key]) || array_key_exists($key, $this->items);
    }
public function containsKey ( $ key ) : bool { return isset ( $ this -> items [ $ key ] ) || array_key_exists ( $ key , $ this -> items ) ; }
Test is the key exists.
Test is the key exists .