This is a common problem I see in failed mod installs: incorrectly renaming files you wish to keep as backups. Zen Cart automatically loads all files ending with the extension “.php” in certain directories (such as includes/functions/extra_functions and includes/modules/order_totals), so you must name your backup files in a way such that they do not end with the extension “.php”.
For example: suppose you want to save a backup copy of the file ot_coupon.php
Good names:
ot_coupon.php.5Oct10
ot_coupon.php.orig
ot_coupon.php.old
Bad Names
5Oct10.ot_coupon.php
orig-ot_coupon.php
old.ot_coupon.php