I have a bunch of client point of sale (POS) systems that periodically send new sales data to one centralized database, which stores the data into one big database for repor
Changing the memory_limit
by ini_set('memory_limit', '-1');
is not a proper solution. Please don't do that.
Your PHP code may have a memory leak somewhere and you are telling the server to just use all the memory that it wants. You wouldn't have fixed the problem at all. If you monitor your server, you will see that it is now probably using up most of the RAM and even swapping to disk.
You should probably try to track down the offending code in your code and fix it.