Your app requires a properly configured Google Cloud Storage bucket. Fortunately, each app can easily gain access to such a bucket. This bucket, called the default bucket, needs no further configuration, permissions, sign-up, or activation. Moreover, the default bucket has a free quota, so you do not need to make your app billable.
For apps created after the App Engine 1.9.0 release, the default bucket is automatically created with your app.
For apps created before the App Engine 1.9.0 release, you can obtain a a default bucket for it by clicking Create within the Cloud Integration section in the Application Settings page of the App Engine Admin Console.
You don't actually need to know your bucket name: you can use
#default# and this
will be replaced at runtime by the name of the default bucket.
For example, you would write to the default bucket using the normal Cloud Storage stream wrapper:
<?php file_put_contents('gs://#default#/hello.txt', 'Hello');
<?php $fp = fopen('gs://#default#/some_file.txt', 'w'); fwrite($fp, 'Hello'); fclose($fp);