Once you get familiar with PHP and have a good grasp on functions and classes, references and syntax - then what? You might find yourself wondering what else there is in the PHP world, what else you can learn. Well, the next step (and this goes for any good language as well) is to learn "code design". Too many developers have just stopped at the "I know the language, that's good enough" stage, and not moved on. Unfortunately for them, this is when the fun really starts. Code theory, software design, and optimization come into play, and this article from IBM talks about a few really good ideas for ways to think to help you take that next step.

Develop rock-solid code in PHP, part 1 goes through a few ideas and thoughts that you should start having when you get past the basics of PHP - things like "never think in pages" and "separate code, content, and presentation". There are some really good ideas in here. I just wish more programmers out there would read it and take it to heart.... (Thanks to PHPEverywhere for the link)