1.php

<?php

$formView = $form->createView();
$renderedFormView = $formRenderer->searchAndRenderBlock($formView, 'rest'); // Object graph of FormRepresentation and InputRepresentation

$content = $this->get('serializer')->serialize($renderedFormView, $format);

2_form.xml

<user>
    <form xmlns="http://www.w3.org/1999/xhtml" method="GET" action="/app_dev.php/users">
        <input type="integer" value="1" name="search[page]"/>
        <input type="integer" value="20" name="search[limit]"/>
    </form>
<user>

3_form.json

{
  "forms" : [
    {
      "action" : "/app_dev.php/users",
      "inputs" : [
        {
          "name" : "search[page]",
          "type" : "integer",
          "value" : "1"
        },
        {
          "name" : "search[limit]",
          "type" : "integer",
          "value" : "20"
        }
      ],
      "method" : "GET",
      "textareas" : [],
      "xmlns" : "http://www.w3.org/1999/xhtml"
    }
  ]
}