On the SitePoint PHP blog a new tutorial introduces you to collection classes in PHP, replacing the more basic array with something with a bit more power.
Applications frequently have objects that contain a group of other objects, and this is a great place to make use of collections. [...] A Collection class is an OOP replacement for the traditional array data structure. Much like an array, a collection contains member elements, although these tend to be objects rather than simpler types such as strings and integers.
He mentions some of the common problems with arrays (and the data they contain) and points out that the structure a "Collection" class wraps around it can help keep things sane. He includes an example of a basic collection class that adds/gets/deletes items from an internal (private) array. He fleshes out this class with code inside those methods and a few others: keys, length and keyExists.