On his blog today Andrey Hristov has a quick post that talks about two methods to extend the functionality that the mysqli extension offers in PHP with your own custom code.

Have you ever though about extending mysqli's classes. It's pretty simple to subclass the connection class mysqli. However, subclassing mysqli_result and mysqli_stmt is not so obvious, actually I though that it's even not possible. However, never say never! After discussing mysqli's OO interface for an hour yesterday [...] I found out how one can plug his own classes.

Two code examples are provided - one for extending the mysqli_result and the other extending the mysql_stmt class. You can find more about these and other classes the mysqli extension has to offer in this section of the PHP manual.