I must not have had sufficient coffee in my bloodstream when reading this, as the first few times I read through it I stumbled on overlooked the fact that % returns a false value when the year is appropriately divisible. Thanks to Corion for smacking me with a the clue-by-four of understanding.

As moritz stated in the CB, think of it as foo() if (($a % X) == 0)

Posted to hasten others to the "oh yeah" moment.