PHP Calculator Table

You can do this:

function getClosest(el, tagName) {
    while (el = el.parentNode) {
        if (el.tagName.toLowerCase() == tagName) return el;
    }
}

window.addEventListener('keyup', function (e) {
    // caso não seja o input pretendido
    if (e.target.name != 'qtd' || e.target.tagName != 'INPUT') return;

    // ir buscar o valor inserido
    var value = parseFloat(e.target.value);

    // ir buscar o outro valor dentro da mesma linha
    var tr = getClosest(e.target, 'tr');
    var valor = tr.querySelector('input[name="valor"]');

    // calcular a soma
    var sum = value + parseFloat(valor.value);

    // inserir a soma no input total da mesma linha
    tr.querySelector('input[name="total"]').value = sum;
});

Example: link

I left comments in the code. The getClosest() function is to go up in the DOM and fetch tr to fetch later inputs that are on the same line.

Notice that I added in the HTML name="valor" in the middle input. This makes it easier and simpler to find this input.

I put the event dropper in window because I have the feeling that this table is dynamic and saaim do not have to worry about delegation.

It's a little confusing, but I think I understand.

First, for multiple inputs, you put it like this:

<input type="text" name="qtd[]" />

Then you retrieve the values again within your while to add to what you need:

<?

$cmd = "SELECT * FROM cotacao";      
$produtos = mysql_query($cmd);
$total = mysql_num_rows($produtos);

$i = 0; //inicia o contador    

while ($linha = mysql_fetch_array($produtos)) {

    $valor = $linha['valor'];
    $data_abertura = $linha['data_abertura'];
    $cod = $linha['cod'];

    $resultado = $_POST["qtd"][$i] + $valor; // aqui ta o resultado
    $i++; //incrementa o contador para pegar o próximo valor

?>

Then you display the result:

<input type="text" name="total" value="<?echo $resultado?>" />