Gonzalo Ayuso came across a situation where he needed to create a factory method inside of multiple classes but didn't want to have to create them each time. With class inheritance there had to be another way.

I have built a backend library. I have a tree of classes and i want to use singleton and factory patterns to my class set. Easy isn’t it? [...] Now imagine you have a lot of classes. You must create over and over the factory function in every classes.

An abstract class is a perfect fit for this sort of thing, but there's not a good way to return the correct kind of object back from the factory method. That is, until you do some magic with a PHP installation version 5.3 and higher. With the get_called_class function, you can easily figure out where the request came from and return a correct object instance with one line.