private function __construct() {
        try {
            $db_config = parse_ini_file(INCLUDE_PATH_CONFIG . 'db_config.ini');

            $dsn = "mysql:host={$db_config['host']};dbname={$db_config['db']};charset={$db_config['charset']}";
            $options = [
                PDO::MYSQL_ATTR_INIT_COMMAND => "SET SESSION sql_mode = ''",
                PDO::MYSQL_ATTR_FOUND_ROWS => true
            ];

            $this->_pdo = new PDO($dsn, $db_config['username'], $db_config['password'], $options);
            $this->_pdo->setAttribute(PDO::ATTR_ORACLE_NULLS, PDO::NULL_NATURAL);
            unset($db_config);
        } catch(PDOException $e) {
            die($e->getMessage());
        }
    }