JS fetch FormData [ $_POST ]

<?php
  // JS fetch FormData [ HTML ]
  // name: add_numbers.php
  // https://pastebin.com/0Rwwe5eT

  if ($_SERVER["REQUEST_METHOD"] == "POST") {
    $number1 = $_POST['one'] ?? null;
    $number2 = $_POST['two'] ?? null;

    if ($number1 && $number2) {
      if (is_numeric($number1) && is_numeric($number2))
        echo ($number1 + $number2);
    }
  }
?>