I’ve run across a somewhat peculiar behavior in PHP that has me scratching my head. I’m genuinely not sure if this is intended behavior, or a bug. I was hoping someone might explain why it behaves the way it does.

So, of course, PHP’s DateTime class is overall pretty good at parsing date strings in a variety of different formats:

echo (new DateTime('today'))->format('Y-m-d'); // Gives 2024-08-15
echo (new DateTime('2012-02-18'))->format('Y-m-d'); // Gives 2012-02-18
echo (new DateTime('2012-02'))->format('Y-m-d'); // Gives 2012-02-01
echo (new DateTime('5 may'))->format('Y-m-d'); // Gives 2024-05-05
echo (new DateTime('6 days ago'))->format('Y-m-d'); // Gives 2024-08-09

However, when passed only a year, for some reason it gives today’s date:

echo (new DateTime('2012'))->format('Y-m-d'); // Gives 2024-08-15

At first, I thought that perhaps it just defaults to ‘now’ when passed an invalid date, or a date in an unexpected format, but that does not appear to be the case. All of the following throw errors:

echo (new DateTime('fart'))->format('Y-m-d');
echo (new DateTime('1'))->format('Y-m-d');
echo (new DateTime('last may'))->format('Y-m-d');
echo (new DateTime('2012-02-48'))->format('Y-m-d');

So, why does passing a bare year give today’s date? Based on PHP’s behavior on other cases, I would expect it to either return Jan 1 of the year in question, or to throw an error. But returning today’s date seems like a very weird choice.

(Note: I’m not looking for a way to get Jan 1 for an arbitrary year; I’m just curious why PHP is behaving this particular way.)