Inner Classes are contained in parent classes and offer a sort of namespace mechanism for the public and open private class data transfer for private subclasses.

The functionality is already available in other languages, but the question is whether it is needed in PHP. The short answer is no. With PHP execution model, it would further slow down and bloat PHP compilation.

He demonstrates their purpose with some simple examples but also gives the reasons why it would be a bad thing to try to include them, including the better choice of namespaces over inner classes.