Seclists.org has posted an advisory titled "Weak RNG in PHP session ID generation leads to session hijacking." RNG stands for Random Number Generation, and the advisory is warning that not enough entropy is being used to seed the RNG; this, in turn, can lead to a reduced number of possible session IDs under certain specific conditions, thus making brute force session spoofing easier, if not bringing it into the realm of feasibility.
The threat is only marked at a "medium" level of severity, but it could still cause problems if you're not careful. There are specific conditions that have to be in place for it to be a problem including using the standard PHP session generation and that the app shares the information about remote users. To protect yourself you can either install Suhosin and don't use the result of set an external source for entropy to help randomize the session ID even more.