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