This indicates an attack attempt to exploit a Buffer Overflow vulnerability in PHP.
The vulnerability is due to an error when the vulnerable function handles a malformed string in a HTTP request. A remote attacker may be able to exploit this to execute arbitrary code within the context of the application, via a crafted HTTP request.
PHP Group PHP prior to 5.4.16
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the most recent upgrade or patch from the vendor.