Processing POST method php

<?php
   //----- (1) LOGIC -----
   $firstName = filter_input(INPUT_POST, 'firstName');
   ?>
<!-- (2) HTML template output -->
<!doctype html>
<html>
   <head>
      <title>process</title>
   </head>
   <body>
      Hello <?= $firstName ?>
   </body>
</html>