The __PHP_Incomplete_Class function in ext/standard/incomplete_class.c in PHP before 5.4.40, 5.5.x before 5.5.24, and 5.6.x before 5.6.8 allows remote attackers to cause a denial of service (application crash) or possibly execute arbitrary code via an unexpected data type, related to a "type confusion" issue.
http://www.securitytracker.com/id/1032709
http://www.securityfocus.com/bid/75249
http://www.oracle.com/technetwork/topics/security/linuxbulletinjan2016-2867209.html
http://www.openwall.com/lists/oss-security/2015/06/16/12
http://rhn.redhat.com/errata/RHSA-2015-1218.html
http://rhn.redhat.com/errata/RHSA-2015-1187.html
http://rhn.redhat.com/errata/RHSA-2015-1186.html
http://rhn.redhat.com/errata/RHSA-2015-1135.html
http://php.net/ChangeLog-5.php
http://git.php.net/?p=php-src.git%3Ba=commit%3Bh=fb83c76deec58f1fab17c350f04c9f042e5977d1