'MongoConnectionException' with message 'No candidate servers found'

mongodb & PHP: Uncaught exception 'MongoConnectionException' with message 'No candidate servers found'
While playing with mongodb, i got the following error in PHP:

2013/03/04 16:47:27 [error] 1399#0: *376172 FastCGI sent in stderr: "PHP message: PHP Fatal error: Uncaught exception 'MongoConnectionException' with message 'No candidate servers found' in /not/relevant/mongoDB.php:28
Stack trace:
#0 /not/relevant/mongoDB.php(28): Mongo->__construct('mongodb://10.0....', Array)

To fix it, check the following:
1. You don't use localhost/127.0.0.1 as a node in your cluster
2. You have the right IPs in the connect to replica field. Check this again.
3. You are using the MongoClient::RP_SECONDARY_PREFERRED and not the MongoClient::RP_SECONDARY