This indicates an attack attempt against a Remote Code Execution vulnerability in PHPUnit.
The vulnerability, which is located in Util/PHP/eval-stdin.php, can be exploited via a HTTP POST request. A remote attacker may be able to exploit this to execute arbitrary code within the context of the application.
PHPUnit before 4.8.28 and 5.x before 5.6.3
System Compromise: Remote attacker can gain control of vulnerable systems.
Apply the most recent upgrade or patch from the vendor.