Update: This entry seems to be getting a lot of new attention; welcome! The lessons of line length, volume, and density, along with lots of other good design principles, are applied to the Solar Framework for PHP 5. Be sure to give it a look if you’re interested in well-designed PHP code.
When it comes to coding style, there are are various ideas about how you should write the individual lines of code. The usual argument is about “how long should a line of code be”? There’s more to it than that, though. Developers should also take into account line volume (“number of lines”) and line density (“instructions per line”).