Question

$cart = $_SESSION['cart'];
    if ($cart) {
        $items = explode(',',$cart);

            //what should i do next?? the value of $items is 4,4,2,2,2,4

Answer 1

To get array without duplicate use array_unique()

                $cart = $_SESSION['cart'];
                    if ($cart) {
                        $items = explode(',',$cart);
                $unique_items=array_unique($items);  // gives 4,2


    $result_array=array();

        foreach($unique_items as $uni_item)
        {
          $item_occurence_count=0;
           $totalvalue=0;

         foreach($items as $item)
            {
            $item_value=$item;
            $totalvalue+=$item_value;
             if($item==$uni_item)
               {
                  ++$item_occurence_count;
                  $result_array[$uni_item]=$item_occurence_count;
               }
            }
        }
print_r($result_array);  // gives Array ( [4] => 3 [2] => 3 ) 

Hope this is what required.