On the Ibuildings techPortal today they've posted the latest episode of their DPC Radio podcast series (as recorded at this year's Dutch PHP Conference). This episode is Derick Rethans' talk on reusability.
This session explain a certain set of design issues for library code that either is often extended, or created with extending and inheritance in mind. There are many ways how a library can be designed to make it both easier to test, as well as for external developers to consume and use. This presentation provides techniques for doing so.