Here are two tutorials from IBM on objects in PHP 5.

The first articles describes the fundamentals of objects and classes in PHP V5, from the very basics through to inheritance, for experienced object-oriented programmers and those who have not yet been introduced to objects.

Link to article: Getting Started with objects in PHP V5 – Why you need to know objects and classes, and how to use them

The second article introduces some of PHP V5′s more advanced and design-oriented features. Among them are object types, which allow for decoupling the components of a system from one another, creating reusable, extensible, and scalable code.

Advanced PHP V5 objects – Create reusable, extensible, and scalable PHP code