It’s easy to find tutorials about the basics of WordPress theme design: the loop, comment forms, template files, etc. If you’re looking to add more advanced functionality to your theme, however, you won’t find help so easily. Sometimes I think commercial theme designers intentionally keep their secrets to themselves…
Caveat: Every time I use words like “complicated” or “advanced,” someone has to complain that none of it is advanced to them. In this case, by “advanced,” I mean that most of the techniques described aren’t the kind of thing you’d find in the Codex.
I first learned about extending WordPress functionality from the original version of this tutorial, the updated version is even better and more useful. Learn how to add custom write panels to posts in WordPress, so your clients can add information and fields like event info, product descriptions, or anything else you can think of.
Custom taxonomies, introduced in WordPress 3.0, can be a little tricky to wrap your head around. This tutorial explains how taxonomies work, why you’d want to use them, and how to implement them in your theme.
If you’re going to be selling a theme commercially, having an options panel is a necessity. Custom options panels let users control all sorts of custom parameters, from stylesheets to categories to whether or not elements are displayed.
This is something of an updated version of the first tutorial here, using Meta Boxes to display custom content instead of custom write panels. The outcome is similar, I recomend reading both to get a good idea of WordPress’ inner workings.
Build Internet is one of my favourite tutorial sites and blogs, because so much care goes into each article, so you’re really learning the why of how things work, instead of just copying and pasting code. This tutorial teaches you how to add custom “add MP3” functionality to your theme.
WordPress 3.0 introduced custom post types, which are a powerful feature that allows for a high level of customization. This tutorial uses the default Twenty Ten theme as a template, and adds custom post support.
No one likes to deal with security issues, but we all know it’s important, especially if you run a high-traffic website. This tutorial goes over the basics of WordPress security and delves into some more advanced techniques.
10. Shameless self-promotion alert: WordPress Featured Content Slider