如何在MongoDB PHP中查找不区分大小写的值？

<?php
$manager = new MongoDB\Driver\Manager("mongodb://localhost:27017");

$filter = ['name' => ['$regex' => new MongoDB\BSON\Regex('^john$', 'i')]];
$options = [];

$query = new MongoDB\Driver\Query($filter, $options);
$cursor = $manager->executeQuery('database.collection', $query);

foreach ($cursor as $document) {
    var_dump($document);
}
?>