In this post to his site Brandon Savage talks about "always returning something" from your methods and functions back to the calling script. He also suggests that null is not an option.

A few weeks ago, there was a discussion on Twitter about whether or not a method should always return a value, or whether or not null was a valid value to return. The answer to this question is a resounding no, a null value should never be returned. [...] For example, you check that a file you opened exists, or that a resource performed correctly before using it. But if you receive a null response, how do you test for this The answer is you can't

He notes that a "null" response is not only difficult to test but can lead to ambiguous handling as you're not sure where the error might be. He also includes a snippet of code showing how a null response could break a fluent interface if an instance of "$this" is not returned.