<?php
header('Content-Type: text/html; charset=utf-8');

if(isset($_GET['summand1'],$_GET['summand2'],$_GET['operand']))
{

function calculate($summand1, $summand2, $operand)
{
  switch($operand)
  {
    case "+":
        $ret =$summand1 + $summand2;
       break;
    case "-":
        $ret = $summand1 -$summand2;
       break;
    case "*":
        $ret = $summand1 * $summand2;
       break;
    case "/":
       $ret = !$summand2
           ? "error"
           : $summand1 / $summand2;
       break;
    default:
       $ret = "error";
  }

  return $ret;
}

$summand1 = filter_input(INPUT_GET, 'summand1');
$summand2 = filter_input(INPUT_GET, 'summand2');
$operand = filter_input(INPUT_GET, 'operand');

echo "$summand1 $operand $summand2 = " . calculate(floatval($summand1), floatval($summand2), $operand);

}else{
?>

    <form action="" method="GET">
        <div>num1:<input type="text" name="summand1" value="10"></div>
        <div>num2:<input type="text" name="summand2" value=""></div>
        <div style="padding: 15px;">
            <br>
            <label>Plus:<input type="radio" name="operand" value="+"></label>
            <label>Minus:<input type="radio" name="operand" value="-"></label>
            <label>Delenie:<input type="radio" name="operand" value="/"></label>
            <label>Umnogenie:<input type="radio" name="operand" value="*"></label>
        </div>
        <div><input type="submit" name="button" value="Button"></div>
    </form>

<?php }?>