php - calculate goat age from its date birth and echo in a field on form -

i have form dynamic select option. want calculate age of goat in months date of birth after selecting db , echo selected goat age on following form field follows dynamic selection option. here selection script:

<?php  $conn = mysqli_connect("localhost", "root", "xxx", "xxxx"); if(mysqli_connect_errno($conn)) { echo "unable connect database server"; }  $sql = "select * goats sex='male'"; $query = mysqli_query($conn, $sql);  echo '<select name="hegoat">'; echo '<option value="">choose goat</option>'; while($hegoats = mysqli_fetch_assoc($query)){ echo "<option>{$hegoats['goatid']}</option>"; } echo '</select>'; ?> 

data base table goats 'dob' column date of birth while form field age is:<input type="text" name="age" id="age"/>

and php code

<?php if(isset($_post['sub']))  {      date_default_timezone_set ("asia/calcutta");     $dateofreg1=date("d m y");     $dbd=$_post['dob'];     $starttimestamp = strtotime($dbd);     $endtimestamp = strtotime($dateofreg1);     $timediff = abs($endtimestamp - $starttimestamp);     $numberdays = $timediff/86400;     $numberdays = intval($numberdays);     $days="total day :".$numberdays;     $birthdate =$dbd;      $birthdate = explode("/", $birthdate);     $age = (date("md", date("u", mktime(0, 0, 0, $birthdate[0], $birthdate[1], $birthdate[2]))) > date("md") ? ((date("y") - $birthdate[2]) - 1) : (date("y") - $birthdate[2]));     $dobs="age is:" . $age;  } ?> 

as per last comment, giving answer.

 <?php if(isset($_post['sub']))      {      date_default_timezone_set ("asia/calcutta");     $dateofreg1=date("d m y");     $dbd=$_post['dob'];     $starttimestamp = strtotime($dbd);     $endtimestamp = strtotime($dateofreg1);      $year1 = date('y', $starttimestamp);   //select year1     $year2 = date('y', $endtimestamp);    //select year 2     $month1 = date('m', $starttimestamp);   //month1     $month2 = date('m', $endtimestamp);      //month2     $diff = (($year2 - $year1) * 12) + ($month2 - $month1);  //year*12 , month difference      $dobs="age is:" . $diff. "  months";      ?>