Zend has a new article posted today as a follow up to their previous "Using ext/mysqli" piece - Using ext/mysqli: Part II - Extending mysqli

The first part of this series provided a quick overview of ext/mysqli. In this installment, we will discuss how to extend the default mysqli class to cleanly provide additional functionality. This brief article is intended for novice programmers who have some experience using the new mysqli extension. It assumes that the basic concepts behind databases and programming are understood, and that readers know how to execute a PHP script and send a query to a MySQL server. Basic familiarity with object-oriented programming will also be useful.

They go on to talk about some of the better points of the mysqli extension, such as its ease of use (thanks to a well-written OOP interface) and the simplicity of extending the class to add aditional features. They also give you two kinds of examples - one simple one that is a "send and fetch" SQL interface, and the other(s) that show you how to add exceptions to your code (both simple and more robust).