MongoClient::_validateReadPreference PHP Method

    public static function _validateReadPreference($readPreference, array $tags = null)
    {
        $new_preference = [];
        if (strcasecmp($readPreference, self::RP_PRIMARY) === 0) {
            if (!empty($tags)) {
                trigger_error("You can't use read preference tags with a read preference of PRIMARY", E_USER_WARNING);
                return false;
            }
            $new_preference['type'] = self::RP_PRIMARY;
        } elseif (strcasecmp($readPreference, self::RP_PRIMARY_PREFERRED) === 0) {
            $new_preference['type'] = self::RP_PRIMARY_PREFERRED;
        } elseif (strcasecmp($readPreference, self::RP_SECONDARY) === 0) {
            $new_preference['type'] = self::RP_SECONDARY;
        } elseif (strcasecmp($readPreference, self::RP_SECONDARY_PREFERRED) === 0) {
            $new_preference['type'] = self::RP_SECONDARY_PREFERRED;
        } elseif (strcasecmp($readPreference, self::RP_NEAREST) === 0) {
            $new_preference['type'] = self::RP_NEAREST;
        } else {
            trigger_error("The value '{$readPreference}' is not valid as read preference type", E_USER_WARNING);
            return false;
        }
        if ($tags) {
            // Also supports string format (dc:east,use:reporting), convert to arrays
            foreach ($tags as $i => $tagset) {
                if (is_string($tagset)) {
                    $array = [];
                    // Empty string can be used to allow no tag matching in the case where
                    // tagsets specified earlier in the array do not match any servers
                    $tagset = $tagset ? explode(',', $tagset) : [];
                    foreach ($tagset as $key_value_pair) {
                        list($key, $value) = explode(':', $key_value_pair);
                        $key = trim($key);
                        $value = trim($value);
                        if ($key === '' || $value === '') {
                            $msg = "Invalid tagset \"{$key_value_pair}\". Must contain non-empty key and value.";
                            trigger_error($msg, E_USER_WARNING);
                            return false;
                        }
                        $array[$key] = $value;
                    }
                    $tags[$i] = $array;
                }
            }
            $new_preference['tagsets'] = $tags;
        }
        return $new_preference;
    }

 /**
  * Set the read preference for this collection
  *
  * @param string $readPreference
  * @param array  $tags
  *
  * @return bool
  */
 public function setReadPreference($readPreference, array $tags = null)
 {
     if ($newPreference = MongoClient::_validateReadPreference($readPreference, $tags)) {
         $this->readPreference = $newPreference;
     }
     return (bool) $newPreference;
 }