PHP before 4.4.3 and 5.x before 5.1.4 does not limit the character set of the session identifier (PHPSESSID) for third party session handlers, which might make it easier for remote attackers to exploit other vulnerabilities by inserting PHP code into the PHPSESSID, which is stored in the session file.  NOTE: it could be argued that this not a vulnerability in PHP itself, rather a design limitation that enables certain attacks against session handlers that do not account for this limitation.