PHP not executing after try block

The reason you are not seeing the HTML content after echo $num is because you are getting a fatal error.

This is seen when running your code in a php console (php -a)

php > $num = new Calculator(1,2,'Subtract');
-1
php > echo $num;
PHP Catchable fatal error:  Object of class Calculator could not be converted to string in php shell code on line 1
PHP Stack trace:
PHP   1. {main}() php shell code:0
php >

To address this problem, we could make a few conceptual changes to your Calculator class:

1. A method should only do One Thing. A constructor is best used for initialization, and your calculator's functions Add, Subtract, Multiply, and Divide probably should be separate methods.
2. As a general rule, an object (being a part of the model, or logic section), should never echo or print its result. This is, in my opinion, a merging of logic and presentation-- which you should strive to separate. It's also part of the problem you ran into.

My first inclination would be to pass the parameters into the methods, instead of the constructor. This allows you to type hint the values you expect. I'm showing integer as example. If bad data (string) is passed, it will throw an exception. If only 1 value is passed, it will throw an exception.

At the end of the day, there's no one "right" way to do it; experience helps show ways that will be easier to work on later. :)

My suggestions, FWIW...

Class:

<?php

class Calculator {

  public function __construct() { }

  public function add(int $addend1, int $addend2) {
    return $addend1 + $addend2;
  }

  public function subtract(int $subtrahend, int $minuend) {
    return $subtrahend - $minuend;
  }

  public function multiply(int $factor1, int $factor2) {
    return $factor1 * $factor2;
  }

  public function divide(int $dividend, int $divisor) {
    return $dividend / $divisor;
  }
}

PHP / HTML

function assignPostVar($name) {
  if(isset($_POST[$name])) {
    return $_POST[$name]; 
  }
}

$error  = '';
if(isset($_POST['submit'])) {
  $calc   = new Calculator;
  $n1     = assignPostVar('number1');
  $n2     = assignPostVar('number2');

  try {
    switch($_POST['submit']) {
      case 'Add': 
        $result = $calc->add($n1,$n2);
        break;

      case 'Subtract': 
        $result = $calc->subtract($n1,$n2);
        break;

      case 'Multiply': 
        $result = $calc->multiply($n1,$n2);
        break;

      case 'Divide': 
        $result = $calc->divide($n1,$n2);
        break;

      default:
        throw new Exception('Invalid operation');            
    }

  } catch (Exception $e) {
    $error = $e->getMessage();
  }
}
?>




<?php if($error): ?>
  <div class="error">Encountered error: <?= $error ?></div>
<?php endif; ?>
<form method="POST">

Number1: <input type="text" name="number1" id="number1" />
<br/>

Number2: <input type="text" name="number2" id="number2" />
<br/><br/>

<input type="submit" id="add" name="submit" value="Add" />

<input type="submit" id="divide" name="submit" value="Divide" />

<input type="submit" id="subtract" name="submit" value="Subtract" />

<input type="submit" id="multiply" name="submit" value="Multiply" />

</form>