如何使用PHP从Firebase中获取父代的子代计数？

composer require kreait/firebase-php

<?php
require 'vendor/autoload.php';

use Kreait\Firebase\Factory;
use Kreait\Firebase\ServiceAccount;

$serviceAccount = ServiceAccount::fromJsonFile('path/to/serviceAccountKey.json');

$firebase = (new Factory)
    ->withServiceAccount($serviceAccount)
    ->create();

$database = $firebase->getDatabase();
?>

<?php
$parentKey = 'parent_key';
$childKey = 'child_key';

$ref = $database->getReference($parentKey)->getChild($childKey);
$count = $ref->getSnapshot()->numChildren();

echo "子代计数：".$count;
?>