Well, easiest thing to do is store the date in the database as a unixtimestamp. This way you can format it as you please without any extra calls later in your php code. Or you can store it in the MySQL format of dates and just reformat it as needed during your SELECT query. I'll show both:
Both ways will assume the following:
<?php
$month = int_val($_POST['month']);
$day = int_val($_POST['day']);
$year = int_val($_POST['year']);
Unix Timestamp
$unixtime = mktime(12,0,0,$month, $day, $year);
MySQL Format
$mysqltime = $year;
$mysqltime .= (strlen($month) < 2) ? '0'.$month : $month;
$mysqltime .= (strlen($day) < 2) ? '0'.$day : $day;
// $mysqltime is now like: YYYYMMDD
Then, it's just a matter of converting it after getting the timestamp back from MySQL into the proper format like:
$date = date('M j, Y', $unixtime);
Or, using the MySQL built-in library to do it for you:
$sql = "SELECT DATE_FORMAT(`timestamp_column`, '%b %e, %Y')
FROM `table_name`";
Hope that helps.
EDIT
Stupid slow university wireless network!!