This indicates an attempt to use Ajax PHP Web Shell.
A web shell is a script that can be uploaded to a web server to enable remote administration of the machine. Installation of a web shell is commonly accomplished through web application vulnerabilities or configuration weaknesses.
Any compromised PHP servers
System Compromise: Remote attacker can gain control of vulnerable systems.
If required, the signature's action can be set to "Block".
Please use Anti-Virus software to scan and clean the system.