您应该使用命名绑定来防止SQL注入，例如：

$results = DB::select('select * from users where id = :id', ['id' => 1]);

参考 www.example.com

可以尝试使用字符串串联运算符'。'将变量值与查询字符串连接起来。例如：

$shift_id = '48c73e3d-57de-4fea-aae4-7222d7e6afed';
    $query = <<<SQL
            SELECT day, date, product1_shift1, product1_shift2, product1_shift3, 
            COALESCE(product1_shift1, 0) + COALESCE(product1_shift2, 0) + COALESCE(product1_shift3, 0) AS total_product1_qty
            FROM (
                SELECT DAYNAME(date) AS day, DATE_FORMAT(date, "%d-%m-%Y") AS date,
                    (SELECT SUM(qty) AS product1_shift1
                    FROM bill_details 
                    JOIN bills ON bills.bill_id=bill_details.bill_id
                    WHERE bills.shift_id='$shift_id'
                    AND bills.date=bills_master.date
                    AND bills_master.customer_id=bills.customer_id
                    AND bill_details.product_id='3516c43b-ffe3-4e90-9d93-924a9865c9cd') AS product1_shift1,
                    (SELECT SUM(qty) AS product1_shift2
                    FROM bill_details 
                    JOIN bills ON bills.bill_id=bill_details.bill_id
                    WHERE bills.shift_id='$shift_id'
                    AND bills.date=bills_master.date
                    AND bills_master.customer_id=bills.customer_id
                    AND bill_details.product_id='3516c43b-ffe3-4e90-9d93-924a9865c9cd') AS product1_shift2,
                    (SELECT SUM(qty) AS product1_shift3
                    FROM bill_details 
                    JOIN bills ON bills.bill_id=bill_details.bill_id
                    WHERE bills.shift_id='$shift_id'
                    AND bills.date=bills_master.date
                    AND bills_master.customer_id=bills.customer_id
                    AND bill_details.product_id='3516c43b-ffe3-4e90-9d93-924a9865c9cd') AS product1_shift3
                FROM bills bills_master
                WHERE bills_master.date BETWEEN '2023-04-07' AND '2023-04-09'
                    AND bills_master.customer_id='d9916741-ca61-4a57-b281-15c80fad833c'
                GROUP BY bills_master.date
            ) subquery_alias
            SQL;
    dd(DB::select($query));

因此，$shift_id使用运算符'连接到查询字符串。看看我能不能帮你。