I really don't like WordPress, and I have worked professionally with it for a long time. My post that consistently gets the most traffic is called "WordPress is stupid", unfortunately it doesn't have good content. Fortunately I moved on from WordPress a few years ago.
With this snippet you can log in a WordPress user automatically. more
See my previous paste for the whole class (minus this) http://kuttler.eu/code/wordpress-plugintheme-options-page/ more
I use helper methods/functions in my themes and plugins to create the input fields, this was simplified for the example here. more
The function should probably check if the post already has the tag/category before adding it a second time. I should probably look into the wpispost_revision() part again as well. more