I can't upload any file to bucket in GCS from App Engine, but I can from my local server coding with php

Question

I have been trying for a long time to upload some invoices that my application (which is running in app engine) creates as pdf, to a bucket in GCS. Later on, the app will send the same by email to each customer. It is working well when running the application in php from my local PC, connecting to App Engine instance. But whenever I am deployin the full code, it does not work at all. I have my credential file for the service account (which the original one created by defaul for my project) in folder in my folder structure of the application. I point to it anytime I am trying to acces the bucket in GCS regardless I am working locally or in GAE. As I say everything goes smooth form local, but nothing from cloud. Here is some code.

//FILE TO UPLOAD TO GCS

date_default_timezone_set('Europe/Madrid');

require __DIR__ . '/../vendor/autoload.php';

require_once __DIR__ . '/../cn.php';

require_once __DIR__ . '/../librerias/fpdf/fpdf.php';

if(!isset($_SESSION)){

        session_start();
 }

use Google\Cloud\Storage\StorageClient;

putenv('GOOGLE_APPLICATION_CREDENTIALS=../librerias/credentials/credentials.json');


function upload_object($bucketName, $objectName, $source){

    $storage = new StorageClient();
    $file = $source;
    $bucket = $storage->bucket($bucketName);
    $object = $bucket->upload($file, [
        'name' => $objectName
    ]);
    printf('Uploaded %s to gs://%s/%s' . PHP_EOL, basename($source), $bucketName, $objectName);
}

//############### CODE GENERATING PDF FILE #######################3

//blablablablablablabla

// ################ END OF PDF FILE GENERATION USING FPDF##################

$pdfData=$pdf->Output('S','',1);
$filename="Facturas/".$ano."/".$factura_1."-".$ano.".pdf";
$bucketName="my-bucket.appspot.com";

upload_object($bucketName, $filename, $pdfData);//THIS IS NOT WORKING WHEM RUNNING FROM GAE, BUT WORKING WELL FROM LOCAL CONNECTING TO MY INSTANCE THROUGH PROXY

//############### CODE SENDING PDF FILE USING SENGRID #######################3

//blablablablablablabla

// ################ END OF PDF FILE SENDING ##################

//END OF FILE UPLOAD TO GCS

//APP.YAML

runtime: php72

runtime_config:
  document_root: .

handlers:
# Serve a directory as a static resource.
- url: /Css
  static_dir: Css
# Serve a directory as a static resource.
- url: /font
  static_dir: font
# Serve a directory as a static resource.
- url: /librerias
  static_dir: librerias
- url: /librerias/credentials
  static_dir: librerias/credentials


# Serve images as static resources.
- url: /icons/(.+\.(gif|png|jpg|jpeg|ico))$
  static_files: icons/\1
  upload: icons/.+\.(gif|png|jpg|jpeg|ico)$
- url: /img/(.+\.(gif|png|jpg|jpeg))$
  static_files: img/\1
  upload: img/.+\.(gif|png|jpg|jpeg)$
- url: /img/Logos/(.+\.(gif|png|jpg|jpeg))$
  static_files: img/Logos/\1
  upload: img/Logos/.+\.(gif|png|jpg|jpeg)$

- url: .*
  script: auto
  secure: always

entrypoint:
  serve handler.php

Answer 1

Solved, I finally got it working. The issue seems to be how to authorize my service account. Instead of

putenv('GOOGLE_APPLICATION_CREDENTIALS=../librerias/credentials/credentials.json');

It is working with

$storage = new StorageClient([
    'keyFile' => json_decode(file_get_contents('mycredentials.json'), true)
]);