Tips on creating a plugin
Security
It is wise to ensure that ALL your PHP plugin files include a line near the top which checks whether IS_ADMIN_FLAG is defined. This way if you have a file that’s accessed via unauthorized methods then it will just abort. For example:
if (!defined('IS_ADMIN_FLAG')) die();
Other variations for specific use in either admin or catalog might check whether it is set to boolean true/false.
Optimizing the use of Overrides
There are built-in override capabilities in Zen Cart to prevent needing to edit some core files which would otherwise need updating.
(As a reminder, the override basics are covered in the storefront help; you may read the introduction, the details, and then the summary.)
Some of the commonly-overlooked override capabilities are listed here:
database_tables.php & filenames.php
Combine your extra details for these two files into one file, and then add it to both the storefront and the admin:
includes/extra_datafiles/my-contribution-name_datafiles.php
admin/includes/extra_datafiles/my-contribution-name_datafiles.php
(these will auto-load)
stylesheet.css
includes/templates/YOURTEMPLATE/css/styles_my-contribution-name.css
(This will auto-load)
Other Resources
Forum Resources
If you run into trouble working on your plugin, you can always post your question on the Contribution-Writing Subforum.