I’ve successfully retrieved the relevant value from the relevant tables. How do I enable those as variables to use within a plugin? Is there an accepted method? There seems to be more than one way to do it.

eg: I can get the thumb URL from one of the gallery tables in the database. How do I get from thumb_url in php to {thumb_url} in EE?

In your PHP, what you’re really doing is looking through the content returned from within the tag and then if some certain string is found (say, “{thumb_url}”) then an action is taken or different content is inserted.

I’d suggest looking at a couple of the Plugins written by either Paul or Rick for examples.