怎样校验输入的日期为合法的日期?这里有一个例子(checkdate.php3)

function is_numeric($var){
   $str = (string)$var;
   $int = (int)$var;
   $strstr = (string)$int;
   if($strstr==$str) return true;
   else return false;
}

function check($year,$month,$day)
{
   if(is_numeric($year) and is_numeric($month) and is_numeric($day)){
      if(checkdate($month,$day,$year)){
         echo $year . "年" . $month . "月" . $day . "日　是合法日期！
";
      }
      else{
         echo $year . "年" . $month . "月" . $day . "日　不是合法日期！
";
      }
   }
   else{
      echo $year . "年" . $month . "月" . $day . "日　不是合法日期！
";
   }
}

check("1999","1","1");
check("2000","2","31");
check("1900","2","1");
check("1900.2","2","1");
check("aaa","1","1");
?>