/**
 * Filters the next, previous and submit buttons.
 * Replaces the form's <input> buttons with <button> while maintaining attributes from original <input>.
 *
 * @param string $button Contains the <input> tag to be filtered.
 * @param object $form Contains all the properties of the current form.
 *
 * @return string The filtered button.
 */
add_filter('gform_submit_button', 'input_to_button', 10, 2);
function input_to_button($button, $form)
{
  $dom = new DOMDocument();
  $dom->loadHTML('<?xml encoding="utf-8" ?>' . $button);
  $input = $dom->getElementsByTagName('input')->item(0);
  $new_button = $dom->createElement('button');
  $new_button->appendChild($dom->createTextNode($input->getAttribute('value')));
  $input->removeAttribute('value');
  foreach ($input->attributes as $attribute) {
    $new_button->setAttribute($attribute->name, $attribute->value);
  }

  $classes = $new_button->getAttribute('class');
  $classes .= " btn is-style-with-arrow";
  $new_button->setAttribute('class', $classes);
  $input->parentNode->replaceChild($new_button, $input);

  return $dom->saveHtml($new_button);
}