php interface vs abstract class
Use an interface when you want to force developers working in your
system (yourself included) to implement a set number of methods on the
classes they'll be building.
Use an abstract class when you want to force developers working in your
system (yourself included) to implement a set numbers of methods and you
want to provide some base methods that will help them develop their child
classes.
Another thing to keep in mind is client classes can only extend one abstract
class, whereas they can implement multiple interfaces. So, if you're
defining your behavior contracts in abstract classes, that means each child
class may only conform to a single contract. Sometimes this a good thing,
when you want to force your user-programmers along a particular path. Other
times it would be bad. Imagine if PHP's Countable and Iterator interfaces
were abstract classes instead of interfaces.
One approach that's common when you're uncertain which way to go (as
mentioned by cletus below) is to create an interface, and then have your
abstract class implement that interface.