$stmt = $mysqli->prepare("SELECT promo_codes FROM promo WHERE promo_codes=? LIMIT 0, 1");
$promo_codes = $_POST["promo"];
$stmt->bind_param('s', $promo_codes);
$stmt->execute();
$result = $stmt->get_result();
if ($promo_code = $result->fetch_row()) {
var_dump($promo_code);
echo 'Промокод найден!';
};