Serendipity is made to be extended, it has an excellent plugin systems woith a lot of hooks in just the right places. There are two kind of plugins in Serendipity at the moment; Sidebar plugins and event plugins. The Sidebar plugins are used to show additional information in the frontend sidebar, the event plugins can do just about anything(except showing something in the sidebar). A combination of the two can be used by specifying dependencies.
Links
The best information is found in excisting plugins. Just browse our cvs reposetorie or Serendipty's.
While trying to get database entries deleted together with the entries wefound out that the "backend_delete_entry" event is not implemented in the latest release(as of Dec 6 2005), the nigtlies do contain this event.