将值插入到相同的id中

I created a quiz and I have problem with answers. When I input the answer, the answer want to insert in to id that is play the game. Instead it creates another id in the table. How can I input multiple answers in different columns but same row?

I tried like this:

if(isset($_POST['button'])) {
    $box = $_POST['opt'];
    $sql = "INSERT INTO gamer (jedan) VALUES ('$box')";
    if(mysqli_query($connsql, $sql)) {
        header('Location:qu2.php');  
    } else {  
        echo'Odgovor nije ubacen!';
   }  
}

You want to use update, not insert. Something like this:

UPDATE gamer
    SET jedan = ?
    WHERE id = ?;

Note that you should be using parameterized queries rather than munging query strings. This is especially important in your case, because the answer to a question could contain all sorts of characters that would break the SQL query.