How can I call form function in html?
Assuming testing() is a page callback, you want this:

function testing()
{
  $form = drupal_get_form('cart_confirmation_form')
  $output = '&lt;div class="popup-related-tabs" style="float: left;" &gt;' . render($form) . '&lt;/div&gt;';

  return $output;
}


Though personally I'd built it as a render array:

function testing()
{
  $page = array
  (
    '#prefix' =&gt; '&lt;div class="popup-related-tabs" style="float: left;" &gt;',
    '#suffix' =&gt; '&lt;/div&gt;',
    'form' =&gt; drupal_get_form('cart_confirmation_form')
  );

  return $page;
}