<?php

function hidden_widget_info() {
  return array(
    'hidden' => array(
      'label' => 'Hidden Field',
      'field types' => array('text'),
    ),
  );
}

function hidden_widget($op, &$node, $field, &$items) {
  switch ($op) {
    case 'form':
      $form = array();

      $form[$field['field_name']] = array('#tree' => TRUE);

      if ($field['multiple']) {
        $delta = 0;
        foreach ($items as $data) {
          if ($data['value']) {
            $form[$field['field_name']][$delta]['value'] = array(
              '#type' => 'hidden',
              '#default_value' => $data['value'],
            );
            $delta++;
          }
        }
        foreach (range($delta, $delta + 2) as $delta) {
          $form[$field['field_name']][$delta]['value'] = array(
            '#type' => 'hidden',
            '#default_value' => '',
          );
        }
      }
      else {
        $form[$field['field_name']][0]['value'] = array(
          '#type' => 'hidden',
          '#default_value' => isset($items[0]['value']) ? $items[0]['value'] : '',
        );
      }
      return $form;

    case 'process form values':
      // Don't save empty fields except the first value
      foreach ($items as $delta => $item) {
        if ($item['value'] == '' && $delta > 0) {
          unset($items[$delta]);
        }
      }
      break;
  }
}