JS fetch FormData [ $_SESSION ]

<?php
  session_start();

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

  if (isset($_COOKIE['PHPSESSID']) && $_SERVER["REQUEST_METHOD"] == "POST") {
    $_SESSION['numbers']['one'] = $_POST['one'] ?? null;
    $_SESSION['numbers']['two'] = $_POST['two'] ?? null;

    if ($_SESSION['numbers']['one'] && $_SESSION['numbers']['two']) {
      if (is_numeric($_SESSION['numbers']['one']) && is_numeric($_SESSION['numbers']['two']))
        echo ($_SESSION['numbers']['one'] + $_SESSION['numbers']['two']);
    }
  }
?>