This indicates an attack attempt to exploit a Memory Corruption vulnerability in PHP.
The vulnerability is due to an error in the application when handling the unserialize() function for specific objects. 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.
PHP Group PHP 5.3.x prior to 5.3.28
PHP Group PHP 5.4.x prior to 5.4.29
PHP Group PHP 5.5.x prior to 5.5.13
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.