Over on PHPMaster.com there's a new post for the unit testers in the audience (you all unit test, right?) from Matt Turland about testing error conditions in your applications.

Let’s say you’re maintaining code that uses PHP’s native trigger_error() function to log error information. Let’s also say that you’re in the process of using PHPUnit to write unit tests for that code. If you refer to the PHPUnit manual, there’s a section that deals with testing for error condition. [...] However, depending on what your code looks like, it’s possible that you’ll run into a problem with PHPUnit’s approach to this. This article will detail what this problem is, how it impacts your ability to test your code, and how to go about solving it.

He points out that, since errors and exceptions handle differently, you have to work with them differently in your tests. PHPUnit has a feature that automatically turns errors into a specific type of exception when they're thrown and how, by using a simple custom error handler, you can more correctly tests error vs exception.