As per your suggestion, I tried looking at options to change the User as bucket name. I couldn't find any option to create a new User in couchbase-cli. Only option to add a user is available when you Add / Edit Node in a cluster.
I also tried below PHP code by changing username to bucket name, but without any success.
$cb = new Couchbase("XXX.XXX.XXX.XXX:8091", "Bucket", "Password", "Bucket", true);
Do let me know, if I'm missing something.