Cart\Cart::totalItems PHP Method

    public function totalItems()
    {
        return array_sum(array_map(function (CartItem $item) {
            return $item->quantity;
        }, $this->items));
    }

 /**
  * @param Cart       $cart
  * @param Translator $i18n
  *
  * @return HtmlTagBuilder
  */
 protected function cartButtonBar($cart, $i18n)
 {
     // Checkout button in Cart Mode
     $checkoutForm = new HtmlTagBuilder('form', array('action' => '', 'method' => 'POST', 'class' => 'checkoutForm'));
     $buttonBar = new HtmlTagBuilder('div', array('class' => 'cartButtonBar'));
     $checkoutBtn = new HtmlTagBuilder('button', array('class' => 'primary', 'name' => 'mode', 'value' => 'checkout', 'type' => 'submit'), array($i18n->translate('button.showCheckout')));
     if ($cart->totalItems() === 0) {
         $checkoutBtn->set('style', 'display:none;');
     }
     $buttonBar->append($checkoutBtn);
     $checkoutForm->append($buttonBar);
     return $checkoutForm;
 }