La requête MySQL fonctionne dans phpmyadmin mais pas dans php (à cause de la date)

vous devriez comparer la date avec la date et non la date avec la date et l'heure

AND  DATE(start_date) <= '2017-08-28' 
AND  DATE(end_date) >= '2017-08-28' 

donc

SELECT id AS couponID, percentage_off, usage_count, used_count 
FROM coupons 
WHERE `code` = 'DANGER' 
AND active_status = '1' 
AND deleted = '0' 
AND  DATE(start_date) <= '2017-08-28' 
AND  DATE(end_date) >= '2017-08-28' 
AND category LIKE '%cLevel%'

dans votre cas, vous utilisez var en php et vous devez convertir correctement en utilisant str_to_date, par exemple :

 str_to_date( '$now'),'%Y-%m-%d');

SELECT id AS couponID, percentage_off, usage_count, used_count 
FROM coupons 
WHERE `code` = 'DANGER' 
AND active_status = '1' 
AND deleted = '0' 
AND  DATE(start_date) <= str_to_date( '$now'),'%Y-%m-%d')
AND  DATE(end_date) >= str_to_date( '$now'),'%Y-%m-%d')
AND category LIKE '%cLevel%'

ou tel qu'utilisé par OP

   str_to_date( '$now'),'%Y-%m-%d %H:%i:%s');

  SELECT id AS couponID, percentage_off, usage_count, used_count 
  FROM coupons 
  WHERE `code` = 'DANGER' 
  AND active_status = '1' 
  AND deleted = '0' 
  AND  start_date <= DATE_FORMAT('$now','%Y-%m-%d %H:%i:%s')
  AND  end_date >= DATE_FORMAT('$now','%Y-%m-%d %H:%i:%s')
  AND category LIKE '%cLevel%'