According to this new advisory on the Hardened-PHP project's site, there are some issues with the Zend Platform product that could cause a number of security issues becaue of malformed session IDs.

During the development of suhosin, which is our new PHP protection module, several compatibility tests with binary 3rd party PHP extensions like the Zend Platform and the Zend Optimizer have been made. When testing the session protection features of suhosin, we discovered that the session clustering system, which is shipping with the Zend Platform is vulnerable to several different attacks.

They mentions a few things a potential attacker could use this issue for, including crashing the session daemon, remote code execution, and being able to view and write files of their choice (like session files) to execute malicious code.

The details are listed out, but a "proof of concept" isn't published for this exploit. Thankfully, Zend has already provided a patch for the issue which can be downloaded at Zend's website (an upgrade to version 2.2.1a).