On the Pixelstech.com site today there's a new post that talks about the differences between abstract classes and interfaces and when's the best time to use either (or both).

When we write programs, we may often get into a situation where we don't know whether we should use Abstract class or Interface when we want to define an abstract object. These two are very similar and they are interchangeable. On Stackoverflow, this question is asked many times, it's related to many programming languages. Also in the official documentation of PHP regarding the Abstract class and Interface, people are arguing about this. To understand this question, we need to understand their differences and use scenarios.

They provide examples of abstract class and interface usage with one of the main differences being that you can define functionality in abstract classes. There's also examples showing classes that extend the abstract class while implementing the interface at the same time, helping to define the object structure to an even finer level.