While testing PHPLIB on Travis CI, I noticed that using "localhost" in a URI instead of "127.0.0.1" leads to a connection timeout:

MongoDB\Driver\ConnectionException: No stream available for server_id 1

The relevant build is: https://travis-ci.org/jmikola/mongo-php-library-prototype/jobs/55846103

This message ultimately comes from the FETCH_FAIL case of libmongoc's mongoc_cluster_fetch_stream() function:

bson_set_error(error,
               MONGOC_ERROR_STREAM,
               MONGOC_ERROR_STREAM_NOT_ESTABLISHED,
               "No stream available for server_id %u", server_id);

We should catch this and use a more user-friendly error message.