Using sessions to pass info between pages

Hi I’m trying to use a session variable to pass a page ID from one page to another – so I know where I’ve come from.
I’m using the shortcode exec php plugin to insert the php

At the start of wp_config.php I’ve put session_start();

In the page I want to pass the page ID from I have:
$_SESSION['pageID'] = get_the_ID();

In the page I want to pass the page ID to I have:

echo "page id =" . $_SESSION['pageID'] . PHP_EOL;

but nothing is passed.

Please can you explain what I’m doing wrong or how best to pass the page ID between pages.

Thanks for any help

David