echo get_age('1988-02-29',date('d'),date('m'),date('Y'));

function get_age($birthday,$d,$m,$y){
    if($birthday == "0000-00-00"){
        return ""; 
    }else{
        $birthday = strtotime($birthday);
        $year = date('Y', $birthday);
        if(($month = ($m - date('m', $birthday))) < 0){
            $year++;
        }else if ($month == 0 && $d - date('d', $birthday) < 0){
            $year++;
        }
        $age = $y - $year;
        return $age;
    }
}