Answer 1

It's probably because of security reasons at sololearn code playground.
1st look at your code:

1. you typed 'output.php' in form action field, but you don't know the local script name
2. correct form : method = "POST"
3. change checking existence of variable (line 26) and reorder 'if' statement:

if(isset($_POST["n1"]) && ($_POST["n1"] != "") && isset($_POST["n2"]) && ($_POST["n2"] != "") && isset($_POST["operation"]))
{
$n1 = doubleval($_POST["n1"]);
$n2 = doubleval($_POST["n2"]);
$operation = $_POST["operation"];
if ($operation == "/" && $n2 == 0)
echo "it is not possible";
else
{
if ($operation == "+") $res = add($n1, $n2);
else if ($operation == "-") $res = sub($n1, $n2);
else if ($operation == "*") $res = mult($n1, $n2);
else $res = divi($n1, $n2);
echo "$n1 $operation $n2 = $res";
}
} else
echo "please fill the two slots";

your script works outside Sololearn Code Playground if script's filename is 'output.php', you can test it on your local computer for example