PDO database connection Class with single static method

PDO database connection Class with single static method

// dbconfig.json
{
    "server": "localhost",
    "dbname": "cl_shop",
    "user" : "root",
    "pass" : "root"
}

// class definition
class DB {

    public static function &conn() {
        $cfg = json_decode(file_get_contents('dbconfig.json', true));
        $conn = NULL;
        if ($conn == NULL) {
            try {
                $conn = new PDO('mysql:host=' . $cfg->server . ';dbname=' . $cfg->dbname, $cfg->user, $cfg->pass);
                $conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
            } catch (PDOException $e) {
                echo 'PDO Error: ' . $e->getMessage();
            }
        }
        return $conn;
    }

}

// usage example
$stm = DB::conn()->prepare('SELECT * FROM users');
$stm->execute();
$res = $stm->fetchAll(PDO::FETCH_CLASS);
var_dump($res);
var_dump($res[0]->id);
var_dump($res[0]->fname);