This indicates an attack attempt to exploit an Uninitialized Reference vulnerability in PHP.
The vulnerability is due to the application attempts to access an uninitialized pointer and a local variable while processing a TAR file. A remote attacker may be able exploit this to execute arbitrary code within the context of a targeted server via a crafted TAR file.
PHP Group PHP prior to 5.5.36
PHP Group PHP prior to 5.6.22
PHP Group PHP prior to 7.0.7
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the most recent upgrade or patch from the vendor.