Can't insert values to MySQL from PHP

Question

I'm using this class for Database operations:

class DataBase
{

    private $connect;

    function __construct()
    {
        $this->connect();
    }

    function connect()
    {
        $this->connect = mysqli_connect(DB_NAME, DB_USER, DB_PASS, DB_NAME);
        if(!$this->connect)
        {
            die('Connecting error: '.mysqli_error());
        }
        else
        {
            $select_db = mysqli_select_db($this->connect, DB_NAME);
            if(!$select_db)
            {
                die('Database not exists!' .mysqli_error());
            }
        }
    }

    function query($sql)
    {
        return mysqli_query($this->connect, $sql);
    }
}

$db = new DataBase();

It works perfect for getting information from DB, but not inserting it. :/

    $connect = mysqli_connect(DB_NAME, DB_USER, DB_PASS, DB_NAME);
    $sql = "INSERT INTO `users` (`login`, `password`, `email`, `name`) VALUES ('$_POST[login]', '$_POST[password]', '$_POST[email]', '$_POST[name]')";
    $db->query($sql);

Even without a class it doesn't insert anything. Wtf?