Hiding the Form After Submission in PHP

Let's say we have a form where two numbers are entered. Let's also assume that after submitting the form, we display the sum of these numbers:

html<form action="" method="GET">
  <input name="test1">
  <input name="test2">
  <input type="submit">
</form>

<?php
  if (!empty($_GET)) {
    echo $_GET['test1'] + $_GET['test2'];
  }
?>

Let's make it so that the form is hidden after submission. To do this, the form code needs to be placed inside a condition:

html<?php
  if (empty($_GET)) {
?>
  <form action="" method="GET">
    <input name="test1">
    <input name="test2">
    <input type="submit">
  </form>
<?php
  } else {
    echo $_GET['test1'] + $_GET['test2'];
  }
?>