Mine

function lastDayOf($month,$year)
{
        while($month < 1)
        {
                $month += 12;
                $year--;
        }

        while($month > 12)
        {
                $month -= 12;
                $year++;
        }

        $months = array(0,31,28,31,30,31,30,31,31,30,31,30,31);
        $num = $months[$month];
        
        if($month == 2)
                if($year%4 == 0)
                        if($year%400 == 0 || $year%100 != 0)
                                $num++;

        return $num;
}