php中的PDO类

Good day! I have a problem of this:

final class Db
{   
    /*
     @ var  object of PDO
    */
    private static $connection = FALSE;

    /*
     @ database settings
    */
    private static $data = [];

    private function __construct()
    {
        self::$data['host'] = 'localhost';
        self::$data['user'] = 'root';
        self::$data['pass'] = '';
        self::$data['db']   = '123';
    }

    /*
     @ method connect to PDO
    */
    private static function connect()
    {
        if(!isset(self::$connection))
        {
            try
            {
                self::$connection = new \PDO('mysql:host='.self::$data['host'].';dbname='.self::$data['db'], self::$data['user'], self::$data['pass']);
                self::$connection -> setAttribute(\PDO::ATTR_ERRMODE, \PDO::ERRMODE_EXCEPTION);
                self::$connection -> exec('SET NAMES utf8');
            }
            catch (PDOException $e)
            {
                die('Ошибка подключения к БД');
            }
        }
    }

    //HERE
    /*
     @ method SQL query in database + return array
     @ param  (string)  $sql            SQL query
     @ param  (array)   $param          Values for execute
     @ param  (const)   $const           PDO Constants for tipization
     @ return (array)
    */
    public function queryFetch($sql, $params = [], $const = PDO::FETCH_ASSOC)
    {
        self::connect();
        return self::$connection -> prepare($sql)-> execute($params) -> fetch($const);
    } 

}

$array = Db::queryFetch('SELECT * FROM `user` WHERE id = ?', ['1']);
var_dump($array);

Fatal error: Call to a member function prepare() on boolean in C:\OpenServer\OpenServer\domains\test1.ru\index.php on line 68
Please if you know what I must to do, help me, It's very important to me

Replace if(!isset(self::$connection)) to if(!self::$connection)

Or replace private static $connection = FALSE; to private static $connection = null;

And read documentation on isset():

isset — Determine if a variable is set and is not NULL