PhpMyAdmin is one of the most popular MySQL administration tools written in PHP. A compromised version of PhpMyAdmin was distributed via a SourceForge mirror and after a security issue was reported, malicious people exploited it. That version had server_sync.php modified and the backdoor is allowing to the attacker to execute PHP code remotely. Because of the fact that SourceForge is the largest open source software development website, if infected many systems. This is not the first time attackers exploit the open source community to distribute their backdoor.
Backdoored PhpMyAdmin from SourceForge