¿Cómo calcular el tiempo restante restante con php y mysql?

Use TIME_TO_SEC(TIMEDIFF(final_time, initial_time))

SELECT TIME_TO_SEC(TIMEDIFF('17:00:00', '09:00:00')) -- 28800

SELECT TIME_TO_SEC(TIMEDIFF('12:30:00', '12:00:00')) -- 1800
SELECT TIME_TO_SEC(TIMEDIFF('10:30:00', '10:15:00')) -- 900