On his Systems Architect site Lukasz Kujawa has posted an introduction to Apache Zookeeper and how to use it in your PHP applications. Zookeeper is "a centralized service for maintaining configuration information, naming, providing distributed synchronization, and providing group services."

Apache ZooKeeper is the coolest technology I recently came across. I found it when I was doing a research about Solr Cloud features. [...] Running multiple application as one logical program is nothing new. In fact creating such a software was one of my first jobs many years ago. This type of architecture is confusing and very tricky to work with. Apache ZooKeeper tries to provide a generic set of tools to manage such a software.

He shows you how to get the PHP extension for Zookepper (along with needed libraries) compiled and installed in your server setup. He includes a sample class that shows you how to connect to the Zookepper server and add a "watcher" your PHP script will respond to when changes happen. The post also has a "Watcher" class example, showing how to use Zookeeper as part of a distributed application. There's some links at the bottom of the post that can also help you get more details on Zookeeper and the PHP interface.