Dynamic sort for mysqli query

Question

I've been trying to get this query to work for a while ... and I just don't know how to build it in order for it to work. The WHERE options are working fine WHERE ".$follow." ".$active." ".$setPub." ".$setLetter." ".$setNumber." ".$setYear." ".$setCond."

But the sort by options are not returning the items in the order that I want.

Is it the way the query is built? Can that be causing the problem? Or is it just the query? I can't see the actual query because I'm using mysqli prepared statements. So I'm a little stuck.

if(array_key_exists('option', $sorts) && $sorts['option'] === "low-price-bids")
        {
            //echo "LOW PRICE BIDS";
            $order = "i.item_start, maxbid ASC";
            $sort = "maxbid IS NOT NULL";
        }

if(array_key_exists('option', $sorts) && $sorts['option'] === "low-price-no-bids")
        {
            //echo "LOW PRICE NO BIDS";
            $order = "i.item_start, maxbid ASC";
            $sort = "maxbid IS NULL";
        }

$q = $this->db->mysqli->prepare("SELECT c.ship_cost, 
                c.item_id, 
                i.id, 
                i.user_id, 
                i.item_title, 
                i.item_number, 
                i.item_year, 
                i.item_publisher, 
                i.item_condition, 
                i.item_start, 
                i.item_description, 
                i.active, 
                u.first_name, 
                u.last_name, 
                CAST(u.fb_id AS CHAR(50)) AS fb_id, 
                u.user_pic, 
                MAX(b.bid) AS maxbid, 
                COUNT(b.bid) AS bids, 
                p.publisher_name
                        FROM countries_ship c
                        JOIN items i
                            ON c.item_id = i.id
                        JOIN users u
                            ON i.user_id = u.id
                        LEFT JOIN bids b
                            ON i.id = b.item_id
                        LEFT JOIN publishers p
                            ON i.item_publisher = p.id
                        WHERE ".$follow." ".$active." ".$setPub." ".$setLetter." ".$setNumber." ".$setYear." ".$setCond." ".$sort."
                        GROUP BY i.id
                        ORDER BY ".$order." LIMIT 18");

Highest price return:

0: {ship_cost: "5", item_id: "624", id: "624", user_id: "62", item_start: "5", maxbid: "",…}
1: {ship_cost: "4", item_id: "623", id: "623", user_id: "62", item_start: "4", maxbid: "",…}
2: {ship_cost: "3", item_id: "622", id: "622", user_id: "296", item_start: "3", maxbid: "10",…}
3: {ship_cost: "2", item_id: "621", id: "621", user_id: "296", item_start: "2", maxbid: "",…}
4: {ship_cost: "0", item_id: "620", id: "620", user_id: "296", item_start: "1", maxbid: "",…}

Lowest price:

0: {ship_cost: "0", item_id: "620", id: "620", user_id: "296", item_start: "1", maxbid: "",…}
1: {ship_cost: "2", item_id: "621", id: "621", user_id: "296", item_start: "2", maxbid: "",…}
2: {ship_cost: "3", item_id: "622", id: "622", user_id: "296", item_start: "3", maxbid: "10",…}
3: {ship_cost: "4", item_id: "623", id: "623", user_id: "62", item_start: "4", maxbid: "",…}
4: {ship_cost: "5", item_id: "624", id: "624", user_id: "62", item_start: "5", maxbid: "",…}