We actually use CSV-reading as an incidental part of a hiring exercise. We provide a really simple homemade CSV parser as part of a PHP project, with a "could you find and fix bugs in this?" instruction. The way to get full marks is to rip out the parser and replace it with the appropriate standard library function.
Only thing that I don't like is that many candidates will assume that they have to fix the code within the parser, given those instructions, even if they know that a battle-tested library is how they would actually do it. I hope you accept an off-hand comment such as, "ew, why is this hand-rolled" as a sufficient indicator in favor of your solution.
