well, you could just try the min and max functions....but they wouldn't let you know which variable is highest or lowest, just the value.
for example, you could try:
max($summernight, $summerweek, $holidaynight, $holidayweek, $winternight, $winterweek, $monthly);
for the max, and
min($summernight, $summerweek, $holidaynight, $holidayweek, $winternight, $winterweek, $monthly);