php > var_dump((new ReflectionClass('MongoDB\Driver\Server'))->getConstants());
array(9) {
  'TYPE_UNKNOWN' =>
  int(0)
  'TYPE_STANDALONE' =>
  int(1)
  'TYPE_MONGOS' =>
  int(2)
  'TYPE_POSSIBLE_PRIMARY' =>
  int(3)
  'TYPE_RS_PRIMARY' =>
  int(4)
  'TYPE_RS_SECONDARY' =>
  int(5)
  'TYPE_RS_ARBITER' =>
  int(6)
  'TYPE_RS_OTHER' =>
  int(7)
  'TYPE_RS_GHOST' =>
  int(8)
}