Расчет возраста в PHP

<?
     //"Дата рождения ".birthday("1980-07-05"); --> Возраст 31

    function birthday ($birthday){
        list($year,$month,$day) = explode("-",$birthday);
        $year_diff  = date("Y") - $year;
        $month_diff = date("m") - $month;
        $day_diff   = date("d") - $day;
        if ($month_diff < 0) $year_diff--;
        elseif (($month_diff==0) && ($day_diff < 0)) $year_diff--;
        return $year_diff;
    }
?>