如何使用PHP代码在GCE上创建凭证（针对谷歌表）？

I put my php code in GCE and wanna to modify google sheets. Google told me that i don't have to apply a extra credential by using GCP>API because there's a strategy called Application Default Credentials (ADC) will find application's credentials. I first check the environment variable "GOOGLE_APPLICATION_CREDENTIALS" in GCE server but it's empty. Then i follow this tutorial https://cloud.google.com/docs/authentication/production?hl=zh_TW#auth-cloud-implicit-php and install this:

composer require google/cloud-storage

I tried the code under and got some error.

namespace Google\Cloud\Samples\Auth;

// Imports GCECredentials and the Cloud Storage client library.
use Google\Auth\Credentials\GCECredentials;
use Google\Cloud\Storage\StorageClient;

function auth_cloud_explicit_compute_engine($projectId)
{
    $gceCredentials = new GCECredentials();
    $config = [
        'projectId' => $projectId,
        'credentialsFetcher' => $gceCredentials,
    ];
    $storage = new StorageClient($config);

    # Make an authenticated API request (listing storage buckets)
    foreach ($storage->buckets() as $bucket) {
        printf('Bucket: %s' . PHP_EOL, $bucket->name());
    }
}

PHP Fatal error: Uncaught Error: Class 'GCECredentials' not found in /var/www/html/google_sheets/t2.php:5

More question: Will this code create a json file as credential for me to access google sheets?

$gceCredentials = new GCECredentials();

Or where can i find the service account key?? Please tell me what should i do, thanks a lot.