if(empty($_COOKIE['user_id'])){
$user_id = openssl_random_pseudo_bytes(10);
setcookie('user_id', $user_id, time()+60*60*24*365*10, '/'); /*работает*/
$query = $conn->prepare('INSERT INTO `comments` (`user_id`) VALUES(:user_id)');
$query->execute(['user_id' => $_COOKIE['user_id']]);
}