PHP 5 before 5.2.7 does not enforce the error_log safe_mode restrictions when safe_mode is enabled through a php_admin_flag setting in httpd.conf, which allows context-dependent attackers to write to arbitrary files by placing a "php_value error_log" entry in a .htaccess file.

More... (http://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2008-5625)