[laimas]

Значит есть ошибки.

--
-- Структура таблицы `L007_demo`
--

CREATE TABLE `L007_demo` (
  `id` int(11) NOT NULL,
  `age` varchar(32) NOT NULL,
  `sex` varchar(32) NOT NULL,
  `stores` varchar(32) NOT NULL,
  `costs` smallint(5) UNSIGNED NOT NULL
) ENGINE=MyISAM DEFAULT CHARSET=utf8;

--
-- Индексы таблицы `L007_demo`
--
ALTER TABLE `L007_demo`
  ADD PRIMARY KEY (`id`);

--
-- AUTO_INCREMENT для сохранённых таблиц
--

--
-- AUTO_INCREMENT для таблицы `L007_demo`
--
ALTER TABLE `L007_demo`
  MODIFY `id` int(11) NOT NULL AUTO_INCREMENT;

<?
$mysqli = mysqli_connect('localhost', 'root', '', 'test');

$stmt = mysqli_prepare($mysqli, "INSERT INTO L007_demo (age, sex, stores, costs) VALUES (?, ?, ?, ?)");
 
if($stmt) {

    $age = '25-35'; $sex = 'female'; $stores = '2 stor'; $costs = 800;
    
    mysqli_stmt_bind_param($stmt, 'sssd', $age, $sex, $stores, $costs);
    
    mysqli_stmt_execute($stmt);
    
    echo mysqli_stmt_affected_rows($stmt) . '<br><pre>';
    
    $q = mysqli_query($mysqli, 'SELECT * FROM L007_demo');
    
    print_r(mysqli_fetch_assoc($q));  

} else exit('error: ' . mysqli_error ($mysqli));

?>

Результат

1

Array
(
    [id] => 1
    [age] => 25-35
    [sex] => female
    [stores] => 2 stor
    [costs] => 800
)