How to loop through sql sum results in PHP?

I need to sum up the result of the total column:

SELECT SUM(number_guest) AS total, booking_time
   FROM mt_bookingtable
   WHERE
            merchant_id IN (SELECT merchant_id 
                              FROM mt_merchant 
                              WHERE merchant_id = 1 )
            AND booking_time BETWEEN "12:00" AND "20:00"
            AND
            date_booking like "2020-06-01"
            AND status in ('pending','approved')
   GROUP BY booking_time

result

I tried to use foreach like this:

foreach ($res as $item) {
    $sum += $item['total'];
    return $sum;
}

But I got the result null.

Solution:

Remove the GROUP BY:

SELECT SUM(number_guest) AS total
FROM mt_bookingtable
WHERE merchant_id IN (SELECT merchant_id FROM mt_merchant WHERE merchant_id = 1 ) AND
      booking_time BETWEEN '12:00' AND '20:00' AND
      date_booking like '2020-06-01' AND
      status in ('pending', 'approved');