WordPress plugins use an event-driven architecture—anyone who has seen a JavaScript function triggered via an HTML «onClick» is familiar with this approach, but in WordPress, the events are typically referred to as hooks. A hook is an event—it is a place where you can attach (or «hook») code. While examining existing plugins, keep an eye out for the add_action() and add_filter() functions that tie a hook to a function inside the plugin. Depending on the author, add_action() and add_filter() instances may be scattered throughout the code or consolidated into one
place.
The number of hooks available in WordPress has been steadily increasing and with version 3, there are well over 1,000. Unfortunately, they are not well documented. This daunting number represents an unwieldy weak point in the WordPress architecture. How can the developer fid the one or two he needs? We have included a list of some of the most common hooks in an appendix at the end of this book.
Many plugins contain convoluted mixtures of logic and HTML within a single function. This scenario is unfortunately common in many PHP CMSs, and it can make it exceedingly diffiult to fid and fi formatting errors. You may be fihting an uphill battle, but we strongly recommend separating your plugin logic from any formatting code. It will make your plugins easier to maintain and skin.