Data in $_SESSION does not update when going to another page

Question

I am trying to store a value from a form into my session, It works ONLY when i update the same page. but when i try to put onclick="window.location='billing.php'" or action="billing.php" in the form. it does not update the value stored in the session.

here are my codes.

<form method="post" name="order">
            <table width="900" border="0" align="center" cellpadding="2" cellspacing="0" id='center'>
                <tr>
                    <td><input type="radio" name="carrier" <?php if (isset($carrier) && $carrier=="LBC") echo "checked";?>  value="LBC"></td>
                    <td><img src="../paymentoptions/LBC.jpg" alt="LBC" class="picture"/></td>
                    <td><p>The Shipping takes 1-2 days for NCR and 2-3 days for any provincial.<p>
                        <div id='price'> Additional ₱250 </div></td>

                </tr>
                <tr>
                    <td><input type="radio" name="carrier" <?php if (isset($carrier) && $carrier=="PickUp") echo "checked";?>  value="PickUp"></td>
                    <td><img src="../paymentoptions/Pick-up.jpg" alt="Pick-Up" class="picture"/></td>
                    <td><p>Office hours: 10:00 am to 5:00 pm<p>
                        <div id='price'> Free!! </div></td>
                </tr>
            </table>
            <br>
            <br>
        <center><h1>Payment Method</h1></center>
        <table width="900" border="0" align="center" cellpadding="2" cellspacing="0" id='centerdown'>
                <tr>
                    <td><input type="radio"  name="payment" <?php if (isset($payment) && $payment=="BPI") echo "checked";?>  value="BPI"></td>
                    <td><img src="../paymentoptions/BPI.jpg"></td>
                    <td><p>Pay by BPI bank deposit (we need confirmation of payment through email.)<p></td>
                </tr>

                <tr>
                    <td><input type="radio"  name="payment" <?php if (isset($payment) && $payment=="PickUp") echo "checked";?>  value="PickUp"></td>
                    <td><img src="../paymentoptions/Pick-up.jpg"></td>
                    <td><p>Pick up. You have 5 days reservation period. You pay for the merchandise upon pick-up<p></td>
                </tr>

            </table>
            <table>
            <tr><td><input type="button" value="View Shopping Cart" onclick="window.location='shoppingcart.php?'"></td><td><input type="submit" name="order" value="Place Order"><!--<input type="button" value="Confirm Order" onclick="window.location='quotation.php?'">--></td></tr>
            </table>
        </form>
<?php 
if (isset($_POST['order'])){
    $_SESSION['carrier'] = $_POST['carrier'];
    $_SESSION['payment'] = $_POST['payment'];
}
?>
    </div>
                <?php echo $_SESSION['carrier']; ?>
                <?php echo $_SESSION['payment']; ?>

how do i fix it so the stored data in the session will also be transfered in the next page? my code right now works but does not go to another page. when i go to another page, the value previously stored in the same page also shows the same value even though i changed the value already.

thank you. hope you can help me.

Answer 1

As per our conversation via chat, am submitting as the answer to close the question.

Your first file has

if (isset($_POST['order'])){
$_SESSION['carrier'] = $_POST['carrier'];
$_SESSION['payment'] = $_POST['payment'];
}

move that to the top of your 2nd file.