In your action page ;
first get date of birth variables.depending on your method $_GET['date_year'] or $_POST['date_year'] etc.Then call mysql_real_escape_string to make them safe for database.($safeYear=mysql_real_escape_string($_GET['date_year']).
Then depending on your database schema call
mysql_query ("insert into youtTable values(....,'$safeDay.$safeMonth.$safeYear',....);
This is the simplest case .If your table has dateTime field than you should pass birth day after formatting obviosuly.By the way I'm assuming you are using your supplied code inside
<form>...</form>
block.
(Sorry for this barbarian answer at 2.00 a.m.)