See an almost exact question and answer
HERE
Your code should then look similar to this -
$query = sprintf(
"SELECT `id`, `maxGuests`
FROM `room`
WHERE `id` NOT IN
(
SELECT `roombooking_room`.`room_id`
FROM `roombooking_room`
JOIN `roombooking` ON `roombooking_room`.`roombooking_id` = `roombooking`.`id`
WHERE `roombooking`.`confirmed` =1
AND (`roomBooking`.`startDate` > DATE(%s) OR `roomBooking`.`endDate` < DATE(%s))
)
AND `maxGuests` <= %d ORDER BY `maxGuests` DESC",
$endDate->toString('yyyy-MM-dd'), $startDate->toString('yyyy-MM-dd'), $noGuests);
$result = $db->query($query);
$result = $result->fetchAll();
$rooms = array();
$guests = 0;
foreach($result as $res) {
if($guests >= $noGuests) break;
$guests += (int)$res['maxGuests'];
$rooms[] = $res['id'];
}