This indicates an attack attempt to exploit a remote Code Execution vulnerability in PHP.
The vulnerability is due to an integer overflow error in the application when handling the unserialize() function. A remote attacker may be able exploit this to execute arbitrary code within the context of the application or possibly cause a denial of service condition via crafted packets.
PHP Group PHP prior to 5.4.34
PHP Group PHP prior to 5.5.18
PHP Group PHP prior to 5.6.2
System Compromise: Remote attackers can gain control of vulnerable systems.
Denial of Service: Remote attackers can crash vulnerable systems.
Apply the most recent upgrade or patch from the vendor.