This indicates an attack attempt to exploit a PHP Code Injection Vulnerability in Horde Framework.
The vulnerability is due to an error when handling a crafted HTTP request. A remote attacker could exploit this to execute arbitrary code execution within the context of the application, via a crafted HTTP request.
Horde Framework 5.1.1 and prior
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the most recent upgrade or patch.