Edit the config.php or config.cluster.php file and set:
define( 'CLUSTER_HEADER_X_POWERED_BY', true );

Check the header of any clustered file. You will see:
X-Powered-By: 1

Edit the config.php or config.cluster.php file and set:
define( 'CLUSTER_HEADER_X_POWERED_BY', false );

Check the header of any clustered file. You will see:
X-Powered-By: the php version