See: https://stackoverflow.com/questions/5593359/where-does-session-save

Summary:

"The session data that you read and write using $_SESSION is stored on server side, usually in text files in a temporary directory. They can not be accessed from outside."

"The session.save_path variable influences the location on the server where the session data is stored."

If you can find where the session is stored you may be able to retrieve the information you need.