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; }