The imagegammacorrect function in ext/gd/gd.c in PHP before 5.6.25 and 7.x before 7.0.10 does not properly validate gamma values, which allows remote attackers to cause a denial of service (out-of-bounds write) or possibly have unspecified other impact by providing different signs for the second and third arguments.
https://www.tenable.com/security/tns-2016-19
https://security.gentoo.org/glsa/201611-22
https://github.com/php/php-src/commit/1bd103df00f49cf4d4ade2cfe3f456ac058a4eae?w=1
http://www.securitytracker.com/id/1036680
http://www.securityfocus.com/bid/92757
http://www.php.net/ChangeLog-7.php
http://www.php.net/ChangeLog-5.php