In case you are trying to add an index to a collection, and the index is already there, the value of getInsertedCount() depends on the MongoDB server version.
For example, MongoDB version 2.4 consider the index as inserted, but version 3.6 does not.
(mongodb >=1.0.0)
MongoDB\Driver\WriteResult::getInsertedCount — Returns the number of documents inserted (excluding upserts)
この関数にはパラメータはありません。
Returns the number of documents inserted (excluding upserts), or null if
the write was not acknowledged.
例1 MongoDB\Driver\WriteResult::getInsertedCount() example
<?php
$manager = new MongoDB\Driver\Manager;
$bulk = new MongoDB\Driver\BulkWrite;
$bulk->insert(['x' => 1]);
$bulk->update(['x' => 1], ['$set' => ['y' => 3]]);
$bulk->update(['x' => 2], ['$set' => ['y' => 1]], ['upsert' => true]);
$bulk->update(['x' => 3], ['$set' => ['y' => 2]], ['upsert' => true]);
$bulk->delete(['x' => 1]);
$result = $manager->executeBulkWrite('db.collection', $bulk);
var_dump($result->getInsertedCount());
?>上の例の出力は以下となります。
int(1)