MongoCollection::setReadPreference PHP Method

    public function setReadPreference($readPreference, $tags = null)
    {
        $result = $this->setReadPreferenceFromParameters($readPreference, $tags);
        $this->createCollectionObject();
        return $result;
    }

 /**
  * Get mongo db stream collection
  *
  * @return \MongoCollection
  */
 private function getCollection()
 {
     if (null === $this->collection) {
         $this->collection = $this->mongoClient->selectCollection($this->dbName, $this->streamCollectionName);
         $this->collection->setReadPreference(\MongoClient::RP_PRIMARY);
     }
     return $this->collection;
 }