如何在SESSION中存储POST值并在另一页PHP上使用它

Question

I have a little newbie question. I've couldn't find proper answer to this, so I have to try here..

Well, I have 2 pages, each one with form on it. 1st page has simple, enter your username, than I want to process it to 2nd one Problem is, when I submit form on 1st page, it seems like it doesn't store my POST data from it, and giving me null value on second page, or empty.. What Ive done so far is.. 1st page

<?php 
session_start();

if (isset($_POST['submit'])) {
    $_SESSION['username']=$_POST['username'];
}
else{
    $username='';
}


 ?>

and form:

<form class="" method="POST" action="combo.php">
                        <label for="username">Enter your username</label>
                        <input type="text" name="username">
                        <?php 
                        print_r($_SESSION); ?>
                        <input type="submit" name="submit" value="Start!">
                    </form>

On second page, when I var_dump('$_SESSION') it gives me a NULL value, so it seems like it isn't stored it from first page.

Can someone please explain what Im doing wrong. Note, on both pages first thing in code is session_start();

Thanks