php interface vs abstract class

1Use an interface when you want to force developers working in your 
2system (yourself included) to implement a set number of methods on the 
3classes they'll be building.
4Use an abstract class when you want to force developers working in your 
5system (yourself included) to implement a set numbers of methods and you 
6want to provide some base methods that will help them develop their child 
7classes.
8Another thing to keep in mind is client classes can only extend one abstract 
9class, whereas they can implement multiple interfaces. So, if you're 
10defining your behavior contracts in abstract classes, that means each child 
11class may only conform to a single contract. Sometimes this a good thing, 
12when you want to force your user-programmers along a particular path. Other 
13times it would be bad. Imagine if PHP's Countable and Iterator interfaces 
14were abstract classes instead of interfaces.
15One approach that's common when you're uncertain which way to go (as 
16mentioned by cletus below) is to create an interface, and then have your 
17abstract class implement that interface.
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