Simple PHP calculator

Question

I\'m creating a basic PHP calculator that lets you enter two values and chose your operator then displays the answer. Everything is working fine except it\'s not outputting

Answer 1

$first = doubleval($_POST['first']);
$second = doubleval($_POST['second']);

if($_POST['group1'] == 'add') {
    echo "$first + $second = ".($first + $second);
}

// etc