How to create a Bearer token for GOOGLE CLOUD JSON API - google-cloud-storage

Question

To upload objects to google cloud storage buckets, I need an authentication token. I have chosen JSON API method to upload objects to buckets.
<?php
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, "https://www.googleapis.com/upload/storage/v1/b/Mybucket/o?uploadType=media");
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($ch, CURLOPT_POSTFIELDS, "yourdata");
curl_setopt($ch, CURLOPT_POST, 1);
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);
$headers = array();
$headers[] = "Authorization:Bearer <TOKEN>";
$headers[] = "x-goog-project-id: xxxxxxxxxxx";
$headers[] = "x-goog-user-project: xxxxxxxxxxx";
$headers[] = "Content-Length: 8";
$headers[] = "Content-Type: image/jpeg";
$headers[] = "x-goog-acl: public-read-write";
curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);
$result = curl_exec($ch);
if (curl_errno($ch)) {
echo 'Error:' . curl_error($ch);
}
curl_close ($ch);
?>
Is there any way to upload object without these type of
authentication or any other authentication method?
How to generate this token (bearer) programmatically using api/curl/php?

Answer 1

function getOAuthToken(){
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, "http://metadata.google.internal/computeMetadata/v1/instance/service-accounts/default/token");
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($ch, CURLOPT_CUSTOMREQUEST, "GET");
$headers = array();
$headers[] = "Metadata-Flavor: Google";
curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);
$result = curl_exec($ch);
if (curl_errno($ch)) {
// echo 'Error:' . curl_error($ch);
}
curl_close ($ch);
$result = json_decode($result,true);
return $result['access_token'];
}
$result['access_token'] is the bearer token.