CVE 2007-1700
The session extension in PHP 4 before 4.4.5, and PHP 5 before 5.2.1, calculates the reference count for the session variables without considering the internal pointer from the session globals, which allows context-dependent attackers to execute arbitrary code via a crafted string in the session_register after unsetting HTTP_SESSION_VARS and _SESSION, which destroys the session data Hashtable.
See the 
CVE page on Mitre.org
for more details.
