In a new post to his blog today Keith Casey talks about something that might be a bit foreign to some PHP developers out there event driven programming. He relate it to the Flex world where it's more commonly used.
When you initially dive into the world of Flex development, most PHP'ers will quickly notice something weird. We're out of the world of Request/Response that we know and understand and into an odd world of Events, Listeners, and Publishers/Subscribers where things just don't play well together.
He talks about the Observer design pattern and how it works to provide an interface to other objects who are listening in and waiting for events to happen (he uses an airline analogy). He also gives a more concrete illustration - uploading a file - and how the events would be used to handle each part of the process.