how to insert three dropdown valuein one field in to database

How can i insert three dropdown values in one field into my database for inserting date of birth. I am $dob variable is it ok or not? please help me.

My form is

 <form name="" id="order-form" class="smart-form" role="form" action="" method="post" enctype="multipart/form-data">
 <tr>
  <td><font color="#334D66"> Date of Birth: </td>
  <td><select name="birthday">
      <option selected></option>
      <option>01</option>
      <option>02</option>
      .
      .
    </select>
    <select name="birthmonth">
      <option selected></option>
      <option>January</option>
      <option>February</option>
      .          
      .
    </select>
    <select name="birthyear">
      <option selected></option>
      <option>1999</option>
      <option>1998</option>
      .
      .
      </select>
  </td>
</tr>

AND my insert code like this

 <?php
if(isset($_POST['submit']))
{

$day=$_POST['birthday'];
$month=$_POST['birthmonth'];
$year=$_POST['birthyear'];
$dob = $day.'/'.$month.'/'.$year;

$data =array('username' => $_POST['username'],
            'firstname' => $_POST['firstname'],
            'lastname' => $_POST['lastname'],
            'email' => $_POST['email'],
            'homepage' => $_POST['homepage'],
            'adress' => $_POST['adress'],
            'city' => $_POST['city'],
            'province_id' => $_POST['province_id'],
            'postcode' => $_POST['postcode'],
            'country_id' => $_POST['country_id'],
            'phone' => $_POST['phone'],
            'dob' => $dob,
            'gender' => $_POST['gender'],
            'occupation' => $_POST['occupation'],
            'fav_quotes' => $_POST['fav_quotes'],
            'news' => $_POST['news'],
            'newsletter' => $_POST['newsletter'],
            'reg_type_id' => $reg_type_id,
            'images' => $_POST['images'] 
            );
insert('user', $data);
//echo "Success";
header("Location: paypal-invoice/createandsend.example.php");
}


?>

Thanks In Advance.

Solution

you can insert dob in a mysql dababase because mysql accept yyyy-mm-dd format so you should

$year.'-'.$month.'-'.$day

I think it will work for you