PHPMaster.com has posted the third part in their "Practical Code Refactoring" series - this time with a focus on Extensibility. (Part 1, Part 2).

Extensible code is a piece of code which follows re-usable, logical, well-known patterns, be it standard design patterns, or normal logical flow. Modular code tends to be highly extensible and monolithic code tends to be non-extensible, but monolithic code might be more efficient, so to solve this conundrum some practices allow developing in a modular way and deploying in a monolithic way so we can get the best of both worlds. The major aspects which we are to discuss with regard to extensible code are: logical extensibility (normal logical flow and design patterns), modular design, and decoupling and encapsulation.

He goes through each of the sections - logical extensibility, modular design and decoupling/encapsulation - and for each provides some questions to ask to help you whip your code into shape.