PHP MongoDB Driver selecting secondary instead of primary mongo db node intermittently - php

Question

I have simple setup with two mongo db nodes (Node A and B). These nodes are also running apache server having php rest api code.
Below are the two scenarios and 2nd is giving problems.
Scenario 1:
1) Node A is mongodb primary and Node B mongodb secondary.
2) REST request sent to Node A, sucessfully processed.
Scenario 2:
1) Node A is mongodb primary and Node B mongodb secondary.
2) REST request sent to Node B, request is processed intermittently.
Scenario 2 logs:
PHP Notice: REPLSET INFO: pick server: random element 0 in /var/www/html/api/lib/mongo.php on line 396
PHP Notice: REPLSET INFO: - connection: type: SECONDARY, socket: 42, ping: 0, hash: secondary.host:27017;-;.;27676 in /var/www/html/api/lib/test.php on line 396
PHP Notice: IO FINE: getting reply in /var/www/html/api/lib/test.php on line 396
PHP Notice: IO FINE: getting cursor header in /var/www/html/api/lib/test.php on line 396
PHP Notice: CON FINE: No timeout changes for secondary.host:27017;-;.;27676 in /var/www/html/api/lib/test.php on line 396
PHP Notice: IO FINE: getting cursor body in /var/www/html/api/lib/test.php on line 396
PHP Notice: CON FINE: No timeout changes for secondary.host:27017;-;.;27676 in /var/www/html/api/lib/test.php on line 396
PHP Notice: CON FINE: mongo_connection_destroy: Destroying connection object for secondary.host:27017;-;.;27676 in /var/www/html/api/lib/test.php on line 396
PHP Notice: CON FINE: mongo_connection_destroy: Closing socket for secondary.host:27017;-;.;27676. in /var/www/html/api/lib/test.php on line 396
PHP Fatal error: Uncaught exception 'MongoCursorException' with message 'secondary.host:27017: not master and slaveOk=false'
For scenario 2, it should always select node A but sometimes it choose node B and fails.
PHP code :
$this->mongo = new \MongoClient ( $list_of_servers, array ('replicaSet' => 'rs0') );
Do I have to make some configuration changes? How to fix this issue?
Thanks
Other information :
MongoDB Version -> 3.0.6
PHP MongoDB Driver -> 1.6.9
PHP version -> 5.3.3
OS -> RedHat