The SoapClient __call method in ext/soap/soap.c in PHP before 5.4.45, 5.5.x before 5.5.29, and 5.6.x before 5.6.13 does not properly manage headers, which allows remote attackers to execute arbitrary code via crafted serialized data that triggers a "type confusion" in the serialize_function_call function.
https://bugs.php.net/bug.php?id=70388
https://security.gentoo.org/glsa/201606-10
http://www.debian.org/security/2015/dsa-3358