Security Enhancements and Fixes in PHP 5.2.4:

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Fixed a floating point exception inside wordwrap() (Reported by Mattias Bengtsson)
Fixed several integer overflows inside the GD extension (Reported by Mattias Bengtsson)
Fixed size calculation in chunk_split() (Reported by Gerhard Wagner)
Fixed integer overflow in str[c]spn(). (Reported by Mattias Bengtsson)
Fixed money_format() not to accept multiple %i or %n tokens. (Reported by Stanislav Malyshev)
Fixed zend_alter_ini_entry() memory_limit interruption vulnerability. (Reported by Stefan Esser)
Fixed INFILE LOCAL option handling with MySQL extensions not to be allowed when open_basedir or safe_mode is active. (Reported by Mattias Bengtsson)
Fixed session.save_path and error_log values to be checked against open_basedir and safe_mode (CVE-2007-3378) (Reported by Maksymilian Arciemowicz)
Fixed a possible invalid read in glob() win32 implementation (CVE-2007-3806) (Reported by shinnai)
Fixed a possible buffer overflow in php_openssl_make_REQ (Reported by zatanzlatan at hotbrev dot com)
Fixed an open_basedir bypass inside glob() function (Reported by dr at peytz dot dk)
Fixed a possible open_basedir bypass inside session extension when the session file is a symlink (Reported by c dot i dot morris at durham dot ac dot uk)
Improved fix for MOPB-03-2007.
Corrected fix for CVE-2007-2872.
For users upgrading to PHP 5.2 from PHP 5.0 and PHP 5.1, an upgrade guide is available here, detailing the changes between those releases and PHP 5.2.4.