It indicates an attempt to execute potentially malicious command via PHPNuke index.php script.
PHPNuke is a website creation/maintenance tool. Due to inadequate input validation, a remote attacker can execute arbitrary code on a target system by passing it a specially-crafted URL.
Any unprotected Francisco Burzi PHP-Nuke 5.3.1 or earlier version is vulnerable to the attack.
Attackers can execute arbitrary code on the victim system, and even gain local access to the system.
Apply appropriate patches or Upgrade the system to the latest non-vulnerable version.