Create multipart form & redirect user

I am fairly new to web design and need some help coding a form. What I am looking to do is take an HTML form that contains a birthday drop down section (mm/dd/yyyy - So 3 different select tabs) and redirect a user based on their age.

If the user is under 30 send them to a new page that says we can't help them. If the user is 30 or over, send them to a second form where they will be able to answer follow up questions... so another form. When processed, I would need to display all the data the user has entered from the beginning of the process.

It's basically 2 form pages, and the 2nd only appears if the user is 30 or over. I am trying to code this using PHP and can't seem to get started. Any help would be appreciated.

Thanks!!

Solution

$interval = date_diff(new DateTime(), new DateTime($_POST['year'] .'-'. $_POST['month'] .'-'. $_POST['day']));

if ($interval->y >= 30)
{
    session_start();
    $_SESSION = $_POST;
    header('Location: over30.php');
}
else
{
    header('Location: under30.php');
}

in the next form...

<?php session_start(); ?>

<html>
    <head><head>
    <body>
        <form method="post" action="xxx.php">
            <input type="text" value="<?=$_SESSION['name'];?>" />
        </form>
    </body>
</html>