php - Cant call multidimensional array -

i trying create cart system ajax , multidimensional session array. array stores product id , quantity of id. trying display quantity of 1 of these stored id's right value not returned.

here php code

public function create()     {         session_start();         $product_id = $_get['data'];          // check if cart has been created         if(isset($_session['cart']) , $_session['cart'] != null)         {             // 1 or more items have been placed in cart         }else{             // cart empty             $_session['cart'] = array();             $_session['cart'][$product_id] = array('quantity' => 3);         }          // return quantity of stored product id         return $_session['cart'][$product_id]['quantity'];     } 

as can see number 3 should returned. instead value 1.