(mongodb >=1.4.0)
MongoDB\Driver\Session::getClusterTime — Returns the cluster time for this session
Returns the cluster time for this session. If the session has not been used
for any operation and
MongoDB\Driver\Session::advanceClusterTime() has not
been called, the cluster time will be null.
This function has no parameters.
Returns the cluster time for this session, or null if the session has no
cluster time.