当var_dump变量...我输出NULL

Database

<?php
    if($cart_id != '')
    {
        $cartQ = mysqli_query($db,"SELECT * FROM `cart`  WHERE id ='$cart_id'");
        $result = mysqli_fetch_assoc($cartQ);
        $items = json_decode($result['items'],true);
        var_dump($items);
        $i = 1;
        $sub_total = 0;
        $item_count = 0;
    }

I'm getting value of var_dump as NULL

I should get the var_dump output as like this

array(1){[0]=> array(3) {['id']=> string(1)"8"["size"]=> string(5)"small"["quantity"]=>string(1)"1"}} 

why it is returning NULL what is the problem in the code?