MongoClient::_getReadProtocol PHP Method

    public function _getReadProtocol(array $readPreference = array())
    {
        if (empty($readPreference)) {
            $readPreference = $this->getReadPreference();
        }
        $this->connect();
        if (!$this->replSet) {
            return reset($this->protocols);
        }
        // Statically cache which protocol is chosen for a given read preference (request association)
        $cache_key = json_encode($readPreference);
        static $cache = [];
        if (isset($cache[$cache_key])) {
            return $cache[$cache_key];
        }
        switch ($readPreference['type']) {
            case self::RP_PRIMARY:
                return $cache[$cache_key] = $this->_getWriteProtocol();
            case self::RP_PRIMARY_PREFERRED:
                try {
                    return $cache[$cache_key] = $this->_getWriteProtocol();
                } catch (MongoConnectionException $e) {
                    // Fall through to reading from secondary
                }
            case self::RP_SECONDARY:
                return $cache[$cache_key] = $this->getNearestHostProtocol([static::STATE_STR_SECONDARY], $readPreference);
            case self::RP_SECONDARY_PREFERRED:
                try {
                    return $cache[$cache_key] = $this->getNearestHostProtocol([static::STATE_STR_SECONDARY], $readPreference);
                } catch (MongoConnectionException $e) {
                    return $cache[$cache_key] = $this->_getWriteProtocol();
                }
            case self::RP_NEAREST:
                return $cache[$cache_key] = $this->getNearestHostProtocol([static::STATE_STR_PRIMARY, static::STATE_STR_SECONDARY], $readPreference);
            default:
                throw new MongoConnectionException("Invalid read preference ({$readPreference['type']}");
        }
    }

 private function fetchMoreDocuments()
 {
     $limit = $this->calculateNextRequestLimit();
     if ($this->end) {
         return;
     }
     $response = $this->client->_getReadProtocol($this->readPreference)->opGetMore($this->fcn, $limit, $this->cursorId, $this->queryTimeout);
     $this->setDocuments($response);
 }