On the Zend Developer Zone there's a recent post from Keith Casey about some of the basics of good OOP design, specifically in following the SOLID principles - a set of five guidelines that make code easier to manage and maintain over time.

As much as we're like to believe that "loose coupling, high cohesion" is enough, when you actually dive into the concept, you find that it's more descriptive than prescriptive. If you want to know how to actually apply these to your day to day development, you have to get into the SOLID principles which describes the five tangible aspects that "good" OO design should contain.

He goes through each of the five principles in the list, describing what they are and, for some, including a bit of sample code to make the point clearer:

• Single Responsibility Principle (SRP)
• Open/Closed Principle (OCP)
• Liskov Substitution Principle (LSP)
• Interface Segregation Principle (ISP)
• Dependency Inversion Principle (DIP)