Failed to create IO Instance error.

This is the php used when getting the error:

<?php
$cb = new Couchbase("localhost:8091", "Administrator", "password", "default");
$cb->set("a", 1);
var_dump($cb->get("a"));
?>



---------------------------
ERROR
----------------------------

PHP Warning: Couchbase::__construct(): failed to create IO instance in /var/www/test_couchbase.php on line 3