<!-- Sends form data to /greeting,
     putting the response onto #result -->
<form action="/greeting#result" target=htmz>
  <label>
    What’s your name?
    <input name="name">
  </label>
  <button>Greet me</button>
</form>

<!-- This will be replaced by /greeting's response -->
<div id="result"></div>

PHP example backend /greeting

<p id="result">
  Hello, <?= $_REQUEST['name'] ?>
</p>