The zend_restore_ini_entry_cb function in zend_ini.c in PHP 5.3.0, 5.2.10, and earlier versions allows context-specific attackers to obtain sensitive information (memory contents) and cause a PHP crash by using the ini_set function to declare a variable, then using the ini_restore function to restore the variable.
http://www.securityfocus.com/bid/36009
http://www.debian.org/security/2009/dsa-1940
http://svn.php.net/viewvc/php/php-src/branches/PHP_5_3/Zend/zend_ini.c?r1=272370&r2=284156