This whitepaper is called Local Session Poisoning in PHP Part 1: The Basics of Exploitation and How to Secure a Server.
You can download it from the following link: https://packetstormsecurity.com/files/download/105783/php-part1.pdf
Source: https://packetstormsecurity.com/files/105783/Local-Session-Poisoning-In-PHP-Part-1.html