<?php
$rc = new MongoDB\Driver\ReadConcern();
var_dump($rc->getLevel());
$rc = new MongoDB\Driver\ReadConcern(MongoDB\Driver\ReadConcern::LOCAL);
var_dump($rc->getLevel());
$rc = new MongoDB\Driver\ReadConcern(MongoDB\Driver\ReadConcern::MAJORITY);
var_dump($rc->getLevel());
?>
The above example will output:
NULL
string(5) "local"
string(8) "majority"