The Registry is one of those Patterns where once you understand it, it seems so incredibly useful. Think of it as a basket; using a Registry you can add data (both values and objects) to the basket, and retrieve them as required from other parts of an application. Since all such data is handled by a single Registry object, it makes passing data and objects around an application far more simple than passing all such values as individual parameters to a constructor or setup method. In a similar fashion, a Registry can replace the need to have every object you need global access to acting as a Singleton.
Source: http://www.patternsforphp.com/index.php?title=Registry