Circuit breaker is a component that supports high relaiability of web sites. It helps discovering, at runtime, which of the external dependencies are failing. Having that knowledge application can avaid wasting time on trying to call them untill they are back online.
Current PHP application may depend on several databases, soap/rest web services, external cache providers or data grids, mail, ftp etc.
It is important for the application to keep on functioning even if some of these dependencies fail. To do that application has to be able to track when services become unavailable and when they become active again.
If a database or web service is down we want our application to detect it as soon as possible and react accordingly. Maybe application has a secondary slave database that can be read from? Maybe there is a way to load cached data? If all fails maybe its best just to hide some function or display message that service will be fully functional soon. What we want to avoid is making every user wait 30s for the database connection (which has to fail any way).
UPDATE: project was moved to github a while back as an independant PHP library php circuit breaker