关于PHP的session调用的问题

请教各位大神，下面PHP代码中"$_SESSION['cart'][$new]"的使用是什么语法？

  if($new) {
    //new item selected
    if(!isset($_SESSION['cart'])) {
      $_SESSION['cart'] = array();
      $_SESSION['items'] = 0;
      $_SESSION['total_price'] ='0.00';
    }

    if(isset($_SESSION['cart'][$new])) {
      $_SESSION['cart'][$new]++;
    } else {
      $_SESSION['cart'][$new] = 1;
    }