How to pass PHP $_SESSION value to new page - php

Question

Im trying to figure out how to use a php $_SESSION value. The situation is:
Main HTML site has an <iframe> that loads products.php
Within products.php a form submits and saves the two values below:
$_SESSION["subtotal"] = $_POST['SUBT'];
$_SESSION["tax"] = $_POST['TAX'];
These work well I can echo them no problem (within that page)
This page replaces products.php with confirmation.php
Once in the confirmation.php I am not able to grab the session values. I have tried the following:
<?php
session_start();
$_SESSION["subtotal"] = $_POST['SUBT'];
$_SESSION["tax"] = $_POST['TAX'];
echo $_SESSION["subtotal"];
echo $_SESSION["tax"];
echo $subtotal;
echo $tax;
?>
AND
$_SESSION["subtotal"] = $subtotal;
$_SESSION["tax"] = $tax;
echo $_SESSION["subtotal"];
echo $_SESSION["tax"];
echo $subtotal;
echo $tax;
?>
AND
$subtotal = $new_subtotal;
$tax = $new_tax;
echo $new_tax;
echo $new_subtotal;
?>
All have failed to see $_SESSION - Is the value getting lost with the <iframe> change?
I have used the following code on the products.php page to confirm the session were susecssfully created - They are:
echo $_SESSION["subtotal"]." stored in session <br />";
echo $_SESSION["tax"]." stored in session <br />";
NOTES:
Both confirmation & products.php are on the same server.
print_r($_SESSION); Returns BLANK array.
session_start() is at the very top of both pages

Answer 1

Use something like this:
products.php
<?php
session_start();
$_SESSION['myValue']=$_GET['YourFormElement'];
//or
$_SESSION['myValue']=3; // You can set the value however you like.
?>
Any other PHP page:
<?php
session_start();
echo $_SESSION['myValue'];
?>
A few notes to keep in mind though: You need to call session_start() BEFORE any output, HTML, echos - even whitespace.
You can keep changing the value in the session - but it will only be able to be used after the first page - meaning if you set it in page 1, you will not be able to use it until you get to another page or refresh the page.
The setting of the variable itself can be done in one of a number of ways:
$_SESSION['myValue']=1;
$_SESSION['myValue']=$var;
$_SESSION['myValue']=$_GET['YourFormElement'];
And if you want to check if the variable is set before getting a potential error, use something like this:
if(!empty($_SESSION['myValue'])
{
echo $_SESSION['myValue'];
}
else
{
echo "Session not set yet.";
}

Answer 2

I did that once by adding $_SESSION to url inside iframe and in file that is used in iframe I used that $_SESSION by $_GET.
I am not sure 100% but it should work as:
$session = $_SESSION['session'];
<iframe src="somePage.php?s=<?php echo $session; ?>"></iframe>
somePage.php
if(isset($_GET['s'])){
$session = $_GET['s'];
}