获取日期范围内的闰日数量

I have the following 2 dates:

$start = new DateTime('2008-06-01');
$end = new DateTime('2016-12-31');
$diff = $start->diff($end);

Now the $diff->days gives 3135 days, but that includes 2 leap days. How should I calculate the amount of February 29's between the $start and $end dates ?

This is for an invoicing system, where leap days should never be counted (company policy)

You can check if it's a leap year:

The only difference between this other question and this one is that where they used $year you write a loop where $i begins at the year you want to start and ends at the end year (: