1) if you’re processing a form that SESSION info - you don’t need to have it in POST to be able to access it. As long as the page that’s processing the form has session_start() - you have acces to that SESSION info and you can then put it in the mail() as needed.

2) if you need to have it in the POST for some reason (maybe you’re processing the form on ANOTHER server - or another service is processing the form) - then when you’re creating the form you can put the SESSION info into hidden input fields in the form and it will be sent along in the POST with the rest of the form…

Hope that makes sense.

That can also be a security risk, depending on how you wrote the rest of the app… such as if you store something like a userID as a sess, and someone posts in a fake form with userID=125, that will now set the sess variable and have them running around as user 125 from now on, with access to that account, or any other account they want to post in as. If you do store key info in a sess that people could potentially guess (or bots post thousands of forms trying to hit something) you won’t want to use that technique.

see function var_dump on the php manual

Then when your ready to use the old post you can either completely overwrite the current post as follows:
$_POST = $_SESSION['previous_post'];

or access that info directly from the session (may have side effects) as follows:
echo $_SESSION['previous_post']['some_posted_variable'];