新文件中的查询结果为空

我编写登录php oop的脚本。我创建了文件Db.php方法runQuery:

    function runQuery($query) {
    $this->last_query_result = $this->conn->query($query);
    if (!$this->last_query_result) {
        $this->setError("Nie udalo sie wykonac zapytania: " . $this->conn->error);
    }
}

在文件login.php中，我创建了新方法loginUser:

    public function loginUser($login, $pass) {
    $this->result = parent::runQuery("SELECT * FROM users WHERE name = '" . $login . "'  AND pass='" . $pass . "'");
    var_dump($this->result);
}

问题是查询返回NULL。

整个文件db.php

class DbUser {
private $conn;
private $host;
private $user;
private $pass;
private $base;
private $last_query_result;
function __construct($host, $user, $pass, $base) {
    $this->host = $host;
    $this->user = $user;
    $this->pass = $pass;
    $this->base = $base;
    $this->connect();
}
public function setError($error) {
    throw new Exception($error);
}
protected function connect() {
    $this->conn = @new mysqli($this->host, $this->user, $this->pass, $this->base);
    if ($this->conn->connect_errno) {
        $this->setError("Nie udalo sie polaczyc z baza danych: " . $this->conn->connect_error);
    }
}
function runQuery($query) {
    $this->last_query_result = $this->conn->query($query);
    if (!$this->last_query_result) {
        $this->setError("Nie udalo sie wykonac zapytania: " . $this->conn->error);
    }
}
function fullList() {
    $return = array();
    while (($row = mysqli_fetch_assoc($this->last_query_result)) !== NULL) {
        $return[] = $row;
    }
    return $return;
}

}

和文件login.php

//kalsa logowania uzytkownika
include 'db.php';
class login extends DbUser {
private $login;
private $pass;
private $result;
function __construct() {
    parent::__construct('localhost', 'michal', 'saba9811', 'test');
}
public function loginUser($login, $pass) {
    $this->result = parent::runQuery("SELECT * FROM users WHERE name = '" . $login . "'  AND pass='" . $pass . "'");
        var_dump($this->result);
    }
}
try {

    $login = $_POST['login'];
    $pass = $_POST['pass'];
    $log = new login();
    $log->loginUser($login, $pass);
} catch (Exception $e) {
    echo $e->getMessage();
}