This indicates an attack attempt to exploit a Stack based Buffer Overflow vulnerability in PHP.
The vulnerability is due to an insufficient boundary checks when the vulnerable application handles malicious requests. A remote attacker may be able exploit this to execute arbitrary codes in the affected machine via crafted requests.
PHP Group PHP 5.4.x prior to 5.4.40
PHP Group PHP 5.5.x prior to 5.5.24
PHP Group PHP 5.6.x prior to 5.6.8
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the most recent upgrade or patch from the vendor.