There is no access counters for _SESSION and HTTP_SESSION_VARS variables, making it possible to trigger use-after-free conditions by unsetting these variables. In addition, it's possible to deserealize these variables.
CVE-2007-1711 (Double free vulnerability in the unserializer in PHP 4.4.5 and 4.4.6 allows context-dependent attackers to execute arbitrary code by overwriting variables pointing to (1) the GLOBALS array or (2) the session data in _SESSION. NOTE: this issue was introduced when attempting to patch CVE-2007-1701 (MOPB-31-2007).)
CVE-2007-1701 (PHP 4 before 4.4.5, and PHP 5 before 5.2.1, when register_globals is enabled, allows context-dependent attackers to execute arbitrary code via deserialization of session data, which overwrites arbitrary global variables, as demonstrated by calling session_decode on a string beginning with "_SESSION|s:39:".)
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.)