Here are a couple more possibilities:
$query = "SELECT YEAR(date_field) as yr,
MONTH(date_field) as mon,
DAY(date_field) as dy
FROM my_table";
or:
$date = strtotime($row['date_field']);
$year = date('Y', $date);
$month = date('m', $date);
$day = date('d', $date);
Myself, though, I'd go with Revraz's for an array or NogDog's for scalars.