In this latest post to his site Matthew Weier O'Phinney gives his suggestion on how to test (unit test) code that provides some kind of direct output. In his case, his script is outputting header information directly, not as a part of a response string.
Here's the scenario: you have code that will emit headers and content, for instance, a front controller. How do you test this? The answer is remarkably simple, but non-obvious: namespaces.
He talks some about the use of namespaces in PHP classes (and methods, and constants...) and how things can be importing using them. He gives an example of an object that outputs some header and body information (an "Output" abstract class). He shows how to use the class in a simple test, calling "reset" in the setup and teardown methods and asserting the contents of the headers and body for expected content.