When connecting to bucket, "global" credentials of authenticator have to be set to target bucket's credentials

<?php

$cluster = new \Couchbase\Cluster('couchbase://192.168.1.194');

$authenticator = new \Couchbase\ClassicAuthenticator();

$authenticator->bucket('protected1', 's3cret');

$authenticator->bucket('protected2', 'password');

$cluster->authenticate($authenticator);

$bucket1 = $cluster->openBucket('protected1');

var_dump($bucket1->upsert('foo', 'bar'));

$bucket2 = $cluster->openBucket('protected2');

var_dump($bucket2->upsert('foo', 'baz'));