Larry Garfield has posted some of his thoughts on return values and reminds you about consistent return types, regardless of the result.
Earlier today, I posted a brief tweet (isn't that redundant?) about return values in PHP (or really, any language). Originally it was about return values from functions (such an exciting topic, I know), but it ended up generating a fair bit of lively conversation, as well as a patch against Drupal 8. So lively, in fact, that I think it deserves more than 140 characters.
He proposes a new rule of thumb: "If your function returns a collection, its null value return must also be a collection." A more broad version of this might be: "make your return types consistent." It's all about predictability and the contracts you have between different parts of your code. If a user calls your method expecting to be able to loop over the results, they'll be disappointed with a "false". He talks some about using and throwing exceptions more effectively for error handling and answers several "but wait..." arguments for his return strategy.