This indicates an attack attempt to exploit a Code Execution vulnerability in PHP.
The vulnerability is due to a use after free error while vulnerable application handles a maliciously crafted serialized object. A remote attacker may be able exploit this to execute arbitrary code within the context of the application via crafted packets.
PHP Group PHP prior to 5.4.37
PHP Group PHP prior to 5.5.21
PHP Group PHP prior to 5.6.5
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the most recent upgrade or patch from the vendor.