แก้ไขวันเดือนปี ให้แสดงข้อมูลกำหนดตามกำหนดตามเงื่อนไข

<select name="day" id="day"  ">
        <option value="">วัน</option>
        <?php
for($i=1;$i<=31;$i++) {
$selected = "";
if (($start_day*1) == $i) $selected = " selected";
echo "<option value='" . str_pad($i, 2, '0', STR_PAD_LEFT) . "'" . $selected . ">" . $i . "</option>";

}

?>
        </select>
        <select name="month" id="month"  ">
          <option value="">เดือน</option>
          <?php
for($i=1;$i<=12;$i++) {
$selected = "";
if (($start_month*1) == $i) $selected = " selected";
echo "<option value='" . str_pad($i, 2, '0', STR_PAD_LEFT) . "'" . $selected . ">" . get_month_name($i) . "</option>";
}
?>
        </select>
        <select name="year" id="year">
          <option value="">ปี</option>
          <?php
for($i=date('Y');$i>=2010;$i--) {
$selected = "";
if (($start_year*1) == $i) $selected = " selected";
echo "<option value='" . $i . "'" . $selected . ">" . ($i+543) . "</option>";

}

?>
      </select>

$day= $_POST['day'];
$month=$_POST['month'];
$year=$_POST['year'];
$date_of_birth=$year."-".$month."-".$day;  ได้เป็นรูปแบบในการเก็บแบบนี้ครับ  2013-5-10