ext/intl/msgformat/msgformat_format.c in PHP before 5.6.26 and 7.x before 7.0.11 does not properly restrict the locale length provided to the Locale class in the ICU library, which allows remote attackers to cause a denial of service (application crash) or possibly have unspecified other impact via a MessageFormatter::formatMessage call with a long first argument.
https://www.tenable.com/security/tns-2016-19
https://security.gentoo.org/glsa/201611-22
https://github.com/php/php-src/commit/6d55ba265637d6adf0ba7e9c9ef11187d1ec2f5b?w=1
https://access.redhat.com/errata/RHSA-2018:1296
http://www.securitytracker.com/id/1036836
http://www.securityfocus.com/bid/93008
http://www.php.net/ChangeLog-7.php