The php_str_replace_in_subject function in ext/standard/string.c in PHP 7.x before 7.0.0 allows remote attackers to execute arbitrary code via a crafted value in the third argument to the str_ireplace function.
http://www.openwall.com/lists/oss-security/2015/07/30/11
http://git.php.net/?p=php-src.git%3Ba=commit%3Bh=6aeee47b2cd47915ccfa3b41433a3f57aea24dd5