If (empty($_SESSION["id"])) {
    session_start();
}
If (!empty($_COOKIE["id"])) {
    $id=$mysql->query("SELECT * FROM users WHERE uid='" . $mysql->real_escape_string($_COOKIE["id"]) . "'");
    $id=$id->fetch_array(MYSQLI_ASSOC);
    If ($id["password"] == $_COOKIE["password"]) {
        setcookie(id, $id["uid"], time() + 60*60*24*30, "/");
        setcookie(password, $id["password"], time() + 60*60*24*30, "/");
        $_SESSION["id"]=$id["uid"];
    }
    Else {
        setcookie("password", "", time() - 3600, "/");
        setcookie("id", "", time() - 3600, "/");
        session_destroy();
    }
}
If (!empty($_POST["username"])) {
    If (!empty($_POST["password"])) {
        $password=md5($_POST["password"]);
        $id=$mysql->query("SELECT uid FROM users WHERE username='" . $mysql->real_escape_string($_POST["username"]) . "' AND password='" . $mysql->real_escape_string($password) . "'");
        If (!empty($id)) {
            $id=$id->fetch_array(MYSQLI_ASSOC);
            setcookie('id', $id["uid"], time() + 60*60*24*30, "/");
            setcookie('password', md5($_POST["password"]), time() + 60*60*24*30, "/");
            $_SESSION["id"]=$id["uid"];
        }
    }
}