php-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;    }