Davey Shafik has taken a look at return values and keeping them standard when handing them back from the results of a database query.
In #phpc we recently had a discussion about function return values; specifically from database queries. I'm going to go on a (admittedly, rather sturdy looking) limb and say this applies to pretty much any function that returns from a data resource, not just a database .
His personal preference is to return the results data if there's matching information but to return a false value if there is an error/not results were found. He includes a snippet of example code to show the structure he's talking about. Some of the comments on the post mention things like exception handling, other similar methods other developers use and the use of nulls.