Buffer underflow in the PHP_FILTER_TRIM_DEFAULT macro in the filtering extension (ext/filter) in PHP 5.2.0 allows context-dependent attackers to execute arbitrary code by calling filter_var with certain modes such as FILTER_VALIDATE_INT, which causes filter to write a null byte in whitespace that precedes the buffer.
http://www.php.net/releases/5_2_1.php
http://www.php-security.org/MOPB/MOPB-19-2007.html
http://www.novell.com/linux/security/advisories/2007_32_php.html
http://www.debian.org/security/2007/dsa-1283