On NetTuts.com today they've continued their series covering the SOLID development principles with the next letter in the acronym - "L". It stands for the Liskov Substitution & Interface Segregation Principles. The tutorial also talks some about the "Interface Segregation Principle" as they go hand-in-hand.
The concept of this principle was introduced by Barbara Liskov in a 1987 conference keynote and later published in a paper together with Jannette Wing in 1994. Their original definition is as follows: "Let q(x) be a property provable about objects x of type T. Then q(y) should be provable for objects y of type S where S is a subtype of T." [or more simply] "Subtypes must be substitutable for their base types."
They include some example PHP code showing a base "Vehicle" class and first an example of doing it correctly (with the Template design pattern) and an example of an incorrect method, complete with tests. They then get into the Interface Segregation Principle, an interface that can be depended on to use the module, with the same car-related examples.