Got it.
Code:
function datediff($start_date, $end_date) {
	list($start_date_month, $start_date_day, $start_date_year) = split('[/.-]', $start_date); 
		if (!$end_date) {
			$end_date_year = date("Y");
			$end_date_month = date("m");
			$end_date_day = date("d");
		} else {
			list($end_date_month, $end_date_day, $end_date_year) = split('[/.-]', $end_date);
		}
			$start_date = mktime(0,0,0,$start_date_month, $start_date_day, $start_date_year);
			$end_date = mktime(0,0,0,$end_date_month, $end_date_day, $end_date_year);
			$difference = $end_date-$start_date;
				return floor($difference/60/60/24);
}