Mikko Koppanen has written up a quick post sharing some work he's done to help make writing extension in PHP a little bit easier. He's created a new project that shares some of his experience in extension development.
I plan to add practical examples related to extension writing. There won't be extensive written documentation outside the code, but hopefully the code itself contains enough nuggets of information to be useful. As the README says, if you need a specific example or clarification on something just open an issue in Github.
The project is still in its early stages, but plans are to have quite a bit more examples as time goes on. Also, as Johannes Schlüter points out in the comments, there's also another example like this - Marcus Boerger's demoext (with accompanying slides).