Oscommerce was written for php version 4. It will work with the new version 5 but needs a code change to be completely compatible. That is what this option changes. You will know if you need this fix because there will be an error that looks like the following:

1064 - You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '-20, 20' at line 1