filter.module

<?php
// $Id$

/**
 * @file
 * Framework for handling filtering of content.
 */

/**
 * Implements hook_help().
 */
function filter_help($path, $arg) {
  switch ($path) {
    case 'admin/help#filter':
      $output = '';
      $output .= '<h3>' . t('About') . '</h3>';
      $output .= '<p>' . t('The Filter module allows administrators to configure text formats. A text format defines the HTML tags, codes, and other input allowed in content and comments, and is a key feature in guarding against potentially damaging input from malicious users. For more information, see the online handbook entry for <a href="@filter">Filter module</a>.', array('@filter' => 'http://drupal.org/handbook/modules/filter/')) . '</p>';
      $output .= '<h3>' . t('Uses') . '</h3>';
      $output .= '<dl>';
      $output .= '<dt>' . t('Configuring text formats') . '</dt>';
      $output .= '<dd>' . t('Configure text formats on the <a href="@formats">Text formats page</a>. <strong>Improper text format configuration is a security risk</strong>. To ensure security, untrusted users should only have access to text formats that restrict them to either plain text or a safe set of HTML tags, since certain HTML tags can allow embedding malicious links or scripts in text. More trusted registered users may be granted permission to use less restrictive text formats in order to create rich content.', array('@formats' => url('admin/config/content/formats'))) . '</dd>';
      $output .= '<dt>' . t('Applying filters to text') . '</dt>';
      $output .= '<dd>' . t('Each text format uses filters to manipulate text, and most formats apply several different filters to text in a specific order. Each filter is designed for a specific purpose, and generally either adds, removes, or transforms elements within user-entered text before it is displayed. A filter does not change the actual content, but instead, modifies it temporarily before it is displayed. One filter may remove unapproved HTML tags, while another automatically adds HTML to make URLs display as clickable links.') . '</dd>';
      $output .= '<dt>' . t('Defining text formats') . '</dt>';
      $output .= '<dd>' . t('One format is included by default: <em>Plain text</em> (which removes all HTML tags). Additional formats may be created by your installation profile when you install Drupal, and more can be created by an administrator on the <a href="@text-formats">Text formats page</a>.', array('@text-formats' => url('admin/config/content/formats'))) . '</dd>';
      $output .= '<dt>' . t('Choosing a text format') . '</dt>';
      $output .= '<dd>' . t('Users with access to more than one text format can use the <em>Text format</em> fieldset to choose between available text formats when creating or editing multi-line content. Administrators can define the text formats available to each user role, and control the order of formats listed in the <em>Text format</em> fieldset on the <a href="@text-formats">Text formats page</a>.', array('@text-formats' => url('admin/config/content/formats'))) . '</dd>';
      $output .= '</dl>';
      return $output;

    case 'admin/config/content/formats':
      $output = '<p>' . t('Text formats define the HTML tags, code, and other formatting that can be used when entering text. <strong>Improper text format configuration is a security risk</strong>. Learn more on the <a href="@filterhelp">Filter module help page</a>.', array('@filterhelp' => url('admin/help/filter'))) . '</p>';
      $output .= '<p>' . t('Text formats are presented on content editing pages in the order defined on this page. The first format available to a user will be selected by default.') . '</p>';
      return $output;

    case 'admin/config/content/formats/%':
      $output = '<p>' . t('A text format contains filters that change the user input, for example stripping out malicious HTML or making URLs clickable. Filters are executed from top to bottom and the order is important, since one filter may prevent another filter from doing its job. For example, when URLs are converted into links before disallowed HTML tags are removed, all links may be removed. When this happens, the order of filters may need to be re-arranged.') . '</p>';
      return $output;
  }
}

/**
 * Implements hook_theme().
 */
function filter_theme() {
  return array(
    'filter_admin_overview' => array(
      'render element' => 'form',
      'file' => 'filter.admin.inc',
    ),
    'filter_admin_format_filter_order' => array(
      'render element' => 'element',
      'file' => 'filter.admin.inc',
    ),
    'filter_tips' => array(
      'variables' => array('tips' => NULL, 'long' => FALSE),
      'file' => 'filter.pages.inc',
    ),
    'text_format_wrapper' => array(
      'render element' => 'element',
    ),
    'filter_tips_more_info' => array(
      'variables' => array(),
    ),
    'filter_guidelines' => array(
      'variables' => array('format' => NULL),
    ),
  );
}

/**
 * Implements hook_element_info().
 *
 * @see filter_process_format()
 */
function filter_element_info() {
  $type['text_format'] = array(
    '#process' => array('filter_process_format'),
    '#base_type' => 'textarea',
    '#theme_wrappers' => array('text_format_wrapper'),
  );
  return $type;
}

/**
 * Implements hook_menu().
 */
function filter_menu() {
  $items['filter/tips'] = array(
    'title' => 'Compose tips',
    'page callback' => 'filter_tips_long',
    'access callback' => TRUE,
    'type' => MENU_SUGGESTED_ITEM,
    'file' => 'filter.pages.inc',
  );
  $items['admin/config/content/formats'] = array(
    'title' => 'Text formats',
    'description' => 'Configure how content input by users is filtered, including allowed HTML tags. Also allows enabling of module-provided filters.',
    'page callback' => 'drupal_get_form',
    'page arguments' => array('filter_admin_overview'),
    'access arguments' => array('administer filters'),
    'file' => 'filter.admin.inc',
  );
  $items['admin/config/content/formats/list'] = array(
    'title' => 'List',
    'type' => MENU_DEFAULT_LOCAL_TASK,
  );
  $items['admin/config/content/formats/add'] = array(
    'title' => 'Add text format',
    'page callback' => 'filter_admin_format_page',
    'access arguments' => array('administer filters'),
    'type' => MENU_LOCAL_ACTION,
    'weight' => 1,
    'file' => 'filter.admin.inc',
  );
  $items['admin/config/content/formats/%filter_format'] = array(
    'type' => MENU_CALLBACK,
    'title callback' => 'filter_admin_format_title',
    'title arguments' => array(4),
    'page callback' => 'filter_admin_format_page',
    'page arguments' => array(4),
    'access arguments' => array('administer filters'),
    'file' => 'filter.admin.inc',
  );
  $items['admin/config/content/formats/%filter_format/delete'] = array(
    'title' => 'Delete text format',
    'page callback' => 'drupal_get_form',
    'page arguments' => array('filter_admin_delete', 4),
    'access callback' => '_filter_delete_format_access',
    'access arguments' => array(4),
    'type' => MENU_CALLBACK,
    'file' => 'filter.admin.inc',
  );
  return $items;
}

/**
 * Access callback for deleting text formats.
 *
 * @param $format
 *   A text format object.
 * @return
 *   TRUE if the text format can be deleted by the current user, FALSE
 *   otherwise.
 */
function _filter_delete_format_access($format) {
  // The fallback format can never be deleted.
  return user_access('administer filters') && ($format->format != filter_fallback_format());
}

/**
 * Load a text format object from the database.
 *
 * @param $format_id
 *   The format ID.
 *
 * @return
 *   A fully-populated text format object.
 */
function filter_format_load($format_id) {
  $formats = filter_formats();
  return isset($formats[$format_id]) ? $formats[$format_id] : FALSE;
}

/**
 * Save a text format object to the database.
 *
 * @param $format
 *   A format object using the properties:
 *   - 'name': The title of the text format.
 *   - 'format': (optional) The internal ID of the text format. If omitted, a
 *     new text format is created.
 *   - 'weight': (optional) The weight of the text format, which controls its
 *     placement in text format lists. If omitted, the weight is set to 0.
 *   - 'filters': (optional) An associative, multi-dimensional array of filters
 *     assigned to the text format, keyed by the name of each filter and using
 *     the properties:
 *     - 'weight': (optional) The weight of the filter in the text format. If
 *       omitted, either the currently stored weight is retained (if there is
 *       one), or the filter is assigned a weight of 10, which will usually
 *       put it at the bottom of the list.
 *     - 'status': (optional) A boolean indicating whether the filter is
 *       enabled in the text format. If omitted, the filter will be disabled.
 *     - 'settings': (optional) An array of configured settings for the filter.
 *       See hook_filter_info() for details.
 */
function filter_format_save(&$format) {
  $format->name = trim($format->name);
  $format->cache = _filter_format_is_cacheable($format);

  // Add a new text format.
  if (empty($format->format)) {
    $return = drupal_write_record('filter_format', $format);
  }
  else {
    $return = drupal_write_record('filter_format', $format, 'format');
  }

  $filter_info = filter_get_filters();
  // Programmatic saves may not contain any filters.
  if (!isset($format->filters)) {
    $format->filters = array();
  }
  foreach ($filter_info as $name => $filter) {
    // Add new filters without weight to the bottom.
    if (!isset($format->filters[$name]['weight'])) {
      $format->filters[$name]['weight'] = 10;
    }
    $format->filters[$name]['status'] = isset($format->filters[$name]['status']) ? $format->filters[$name]['status'] : 0;
    $format->filters[$name]['module'] = $filter['module'];

    // If settings were passed, only ensure default settings.
    if (isset($format->filters[$name]['settings'])) {
      if (isset($filter['default settings'])) {
        $format->filters[$name]['settings'] = array_merge($filter['default settings'], $format->filters[$name]['settings']);
      }
    }
    // Otherwise, use default settings or fall back to an empty array.
    else {
      $format->filters[$name]['settings'] = isset($filter['default settings']) ? $filter['default settings'] : array();
    }

    $fields = array();
    $fields['weight'] = $format->filters[$name]['weight'];
    $fields['status'] = $format->filters[$name]['status'];
    $fields['module'] = $format->filters[$name]['module'];
    $fields['settings'] = serialize($format->filters[$name]['settings']);

    db_merge('filter')
      ->key(array(
        'format' => $format->format,
        'name' => $name,
      ))
      ->fields($fields)
      ->execute();
  }

  if ($return == SAVED_NEW) {
    module_invoke_all('filter_format_insert', $format);
  }
  else {
    module_invoke_all('filter_format_update', $format);
    // Explicitly indicate that the format was updated. We need to do this
    // since if the filters were updated but the format object itself was not,
    // the call to drupal_write_record() above would not return an indication
    // that anything had changed.
    $return = SAVED_UPDATED;

    // Clear the filter cache whenever a text format is updated.
    cache_clear_all($format->format . ':', 'cache_filter', TRUE);
  }

  filter_formats_reset();

  return $return;
}

/**
 * Delete a text format.
 *
 * @param $format
 *   The text format object to be deleted.
 * @param $fallback_id
 *   (optional) The ID of the text format to use to reassign content that is
 *   currently using $format. If omitted, the currently stored
 *   filter_fallback_format() is used.
 */
function filter_format_delete($format, $fallback_id = NULL) {
  db_delete('filter_format')
    ->condition('format', $format->format)
    ->execute();
  db_delete('filter')
    ->condition('format', $format->format)
    ->execute();

  // Allow modules to react on text format deletion.
  if (empty($fallback_id)) {
    $fallback_id = filter_fallback_format();
  }
  $fallback = filter_format_load($fallback_id);
  module_invoke_all('filter_format_delete', $format, $fallback);

  filter_formats_reset();
  cache_clear_all($format->format . ':', 'cache_filter', TRUE);
}

/**
 * Display a text format form title.
 */
function filter_admin_format_title($format) {
  return $format->name;
}

/**
 * Implements hook_permission().
 */
function filter_permission() {
  $perms['administer filters'] = array(
    'title' => t('Administer text formats and filters'),
    'restrict access' => TRUE,
  );

  // Generate permissions for each text format. Warn the administrator that any
  // of them are potentially unsafe.
  foreach (filter_formats() as $format) {
    $permission = filter_permission_name($format);
    if (!empty($permission)) {
      // Only link to the text format configuration page if the user who is
      // viewing this will have access to that page.
      $format_name_replacement = user_access('administer filters') ? l($format->name, 'admin/config/content/formats/' . $format->format) : drupal_placeholder($format->name);
      $perms[$permission] = array(
        'title' => t("Use the !text_format text format", array('!text_format' => $format_name_replacement,)),
        'description' => drupal_placeholder(t('Warning: This permission may have security implications depending on how the text format is configured.')),
      );
    }
  }
  return $perms;
}

/**
 * Returns the machine-readable permission name for a provided text format.
 *
 * @param $format
 *   An object representing a text format.
 * @return
 *   The machine-readable permission name, or FALSE if the provided text format
 *   is malformed or is the fallback format (which is available to all users).
 */
function filter_permission_name($format) {
  if (isset($format->format) && $format->format != filter_fallback_format()) {
    return 'use text format ' . $format->format;
  }
  return FALSE;
}

/**
 * Implements hook_cron().
 *
 * Expire outdated filter cache entries
 */
function filter_cron() {
  cache_clear_all(NULL, 'cache_filter');
}

/**
 * Implements hook_modules_enabled().
 */
function filter_modules_enabled($modules) {
  // Reset the static cache of module-provided filters, in case any of the
  // newly enabled modules defines a new filter or alters existing ones.
  drupal_static_reset('filter_get_filters');
}

/**
 * Implements hook_modules_disabled().
 */
function filter_modules_disabled($modules) {
  // Reset the static cache of module-provided filters, in case any of the
  // newly disabled modules defined or altered any filters.
  drupal_static_reset('filter_get_filters');
}

/**
 * Retrieve a list of text formats, ordered by weight.
 *
 * @param $account
 *   (optional) If provided, only those formats that are allowed for this user
 *   account will be returned. All formats will be returned otherwise.
 * @return
 *   An array of text format objects, keyed by the format ID and ordered by
 *   weight.
 *
 * @see filter_formats_reset()
 */
function filter_formats($account = NULL) {
  $formats = &drupal_static(__FUNCTION__, array());

  // Statically cache all existing formats upfront.
  if (!isset($formats['all'])) {
    $formats['all'] = db_select('filter_format', 'ff')
      ->addTag('translatable')
      ->fields('ff')
      ->orderBy('weight')
      ->execute()
      ->fetchAllAssoc('format');
  }

  // Build a list of user-specific formats.
  if (isset($account) && !isset($formats['user'][$account->uid])) {
    $formats['user'][$account->uid] = array();
    foreach ($formats['all'] as $format) {
      if (filter_access($format, $account)) {
        $formats['user'][$account->uid][$format->format] = $format;
      }
    }
  }

  return isset($account) ? $formats['user'][$account->uid] : $formats['all'];
}

/**
 * Resets the static cache of all text formats.
 *
 * @see filter_formats()
 */
function filter_formats_reset() {
  drupal_static_reset('filter_list_format');
  drupal_static_reset('filter_formats');
}

/**
 * Retrieves a list of roles that are allowed to use a given text format.
 *
 * @param $format
 *   An object representing the text format.
 * @return
 *   An array of role names, keyed by role ID.
 */
function filter_get_roles_by_format($format) {
  // Handle the fallback format upfront (all roles have access to this format).
  if ($format->format == filter_fallback_format()) {
    return user_roles();
  }
  // Do not list any roles if the permission does not exist.
  $permission = filter_permission_name($format);
  return !empty($permission) ? user_roles(FALSE, $permission) : array();
}

/**
 * Retrieves a list of text formats that are allowed for a given role.
 *
 * @param $rid
 *   The user role ID to retrieve text formats for.
 * @return
 *   An array of text format objects that are allowed for the role, keyed by
 *   the text format ID and ordered by weight.
 */
function filter_get_formats_by_role($rid) {
  $formats = array();
  foreach (filter_formats() as $format) {
    $roles = filter_get_roles_by_format($format);
    if (isset($roles[$rid])) {
      $formats[$format->format] = $format;
    }
  }
  return $formats;
}

/**
 * Returns the ID of the default text format for a particular user.
 *
 * The default text format is the first available format that the user is
 * allowed to access, when the formats are ordered by weight. It should
 * generally be used as a default choice when presenting the user with a list
 * of possible text formats (for example, in a node creation form).
 *
 * Conversely, when existing content that does not have an assigned text format
 * needs to be filtered for display, the default text format is the wrong
 * choice, because it is not guaranteed to be consistent from user to user, and
 * some trusted users may have an unsafe text format set by default, which
 * should not be used on text of unknown origin. Instead, the fallback format
 * returned by filter_fallback_format() should be used, since that is intended
 * to be a safe, consistent format that is always available to all users.
 *
 * @param $account
 *   (optional) The user account to check. Defaults to the currently logged-in
 *   user.
 * @return
 *   The ID of the user's default text format.
 *
 * @see filter_fallback_format()
 */
function filter_default_format($account = NULL) {
  global $user;
  if (!isset($account)) {
    $account = $user;
  }
  // Get a list of formats for this user, ordered by weight. The first one
  // available is the user's default format.
  $formats = filter_formats($account);
  $format = reset($formats);
  return $format->format;
}

/**
 * Returns the ID of the fallback text format that all users have access to.
 *
 * The fallback text format is a regular text format in every respect, except
 * it does not participate in the filter permission system and cannot be
 * deleted. It needs to exist because any user who has permission to create
 * formatted content must always have at least one text format they can use.
 *
 * Because the fallback format is available to all users, it should always be
 * configured securely. For example, when the Filter module is installed, this
 * format is initialized to output plain text. Installation profiles and site
 * administrators have the freedom to configure it further.
 *
 * Note that the fallback format is completely distinct from the default
 * format, which differs per user and is simply the first format which that
 * user has access to. The default and fallback formats are only guaranteed to
 * be the same for users who do not have access to any other format; otherwise,
 * the fallback format's weight determines its placement with respect to the
 * user's other formats.
 *
 * @see hook_filter_format_delete()
 * @see filter_default_format()
 */
function filter_fallback_format() {
  // This variable is automatically set in the database for all installations
  // of Drupal. In the event that it gets deleted somehow, there is no safe
  // default to return, since we do not want to risk making an existing (and
  // potentially unsafe) text format on the site automatically available to all
  // users. Returning NULL at least guarantees that this cannot happen.
  return variable_get('filter_fallback_format');
}

/**
 * Returns the title of the fallback text format.
 */
function filter_fallback_format_title() {
  $fallback_format = filter_format_load(filter_fallback_format());
  return filter_admin_format_title($fallback_format);
}

/**
 * Return a list of all filters provided by modules.
 */
function filter_get_filters() {
  $filters = &drupal_static(__FUNCTION__, array());

  if (empty($filters)) {
    foreach (module_implements('filter_info') as $module) {
      $info = module_invoke($module, 'filter_info');
      if (isset($info) && is_array($info)) {
        // Assign the name of the module implementing the filters and ensure
        // default values.
        foreach (array_keys($info) as $name) {
          $info[$name]['module'] = $module;
          $info[$name] += array(
            'description' => '',
            'weight' => 0,
          );
        }
        $filters = array_merge($filters, $info);
      }
    }
    // Allow modules to alter filter definitions.
    drupal_alter('filter_info', $filters);

    uasort($filters, '_filter_list_cmp');
  }

  return $filters;
}

/**
 * Helper function for sorting the filter list by filter name.
 */
function _filter_list_cmp($a, $b) {
  return strcmp($a['title'], $b['title']);
}

/**
 * Check if text in a certain text format is allowed to be cached.
 *
 * This function can be used to check whether the result of the filtering
 * process can be cached. A text format may allow caching depending on the
 * filters enabled.
 *
 * @param $format_id
 *   The text format ID to check.
 * @return
 *   TRUE if the given text format allows caching, FALSE otherwise.
 */
function filter_format_allowcache($format_id) {
  $format = filter_format_load($format_id);
  return !empty($format->cache);
}

/**
 * Helper function to determine whether the output of a given text format can be cached.
 *
 * The output of a given text format can be cached when all enabled filters in
 * the text format allow caching.
 *
 * @param $format
 *   The text format object to check.
 * @return
 *   TRUE if all the filters enabled in the given text format allow caching,
 *   FALSE otherwise.
 *
 * @see filter_format_save()
 */
function _filter_format_is_cacheable($format) {
  if (empty($format->filters)) {
    return TRUE;
  }
  $filter_info = filter_get_filters();
  foreach ($format->filters as $name => $filter) {
    // By default, 'cache' is TRUE for all filters unless specified otherwise.
    if (!empty($filter['status']) && isset($filter_info[$name]['cache']) && !$filter_info[$name]['cache']) {
      return FALSE;
    }
  }
  return TRUE;
}

/**
 * Retrieve a list of filters for a given text format.
 *
 * Note that this function returns all associated filters regardless of whether
 * they are enabled or disabled. All functions working with the filter
 * information outside of filter administration should test for $filter->status
 * before performing actions with the filter.
 *
 * @param $format_id
 *   The format ID to retrieve filters for.
 *
 * @return
 *   An array of filter objects associated to the given text format, keyed by
 *   filter name.
 */
function filter_list_format($format_id) {
  $filters = &drupal_static(__FUNCTION__, array());
  $filter_info = filter_get_filters();

  if (!isset($filters['all'])) {
    $result = db_query('SELECT * FROM {filter} ORDER BY weight, module, name');
    foreach ($result as $record) {
      $filters['all'][$record->format][$record->name] = $record;
    }
  }

  if (!isset($filters[$format_id])) {
    $format_filters = array();
    foreach ($filters['all'][$format_id] as $name => $filter) {
      if (isset($filter_info[$name])) {
        $filter->title = $filter_info[$name]['title'];
        // Unpack stored filter settings.
        $filter->settings = (isset($filter->settings) ? unserialize($filter->settings) : array());
        // Merge in default settings.
        if (isset($filter_info[$name]['default settings'])) {
          $filter->settings += $filter_info[$name]['default settings'];
        }

        $format_filters[$name] = $filter;
      }
    }
    $filters[$format_id] = $format_filters;
  }

  return isset($filters[$format_id]) ? $filters[$format_id] : array();
}

/**
 * Run all the enabled filters on a piece of text.
 *
 * Note: Because filters can inject JavaScript or execute PHP code, security is
 * vital here. When a user supplies a text format, you should validate it using
 * filter_access() before accepting/using it. This is normally done in the
 * validation stage of the Form API. You should for example never make a preview
 * of content in a disallowed format.
 *
 * @param $text
 *   The text to be filtered.
 * @param $format_id
 *   The format id of the text to be filtered. If no format is assigned, the
 *   fallback format will be used.
 * @param $langcode
 *   Optional: the language code of the text to be filtered, e.g. 'en' for
 *   English. This allows filters to be language aware so language specific
 *   text replacement can be implemented.
 * @param $cache
 *   Boolean whether to cache the filtered output in the {cache_filter} table.
 *   The caller may set this to FALSE when the output is already cached
 *   elsewhere to avoid duplicate cache lookups and storage.
 *
 * @ingroup sanitization
 */
function check_markup($text, $format_id = NULL, $langcode = '', $cache = FALSE) {
  if (empty($format_id)) {
    $format_id = filter_fallback_format();
  }
  $format = filter_format_load($format_id);

  // Check for a cached version of this piece of text.
  $cache = $cache && !empty($format->cache);
  $cache_id = '';
  if ($cache) {
    $cache_id = $format->format . ':' . $langcode . ':' . hash('sha256', $text);
    if ($cached = cache_get($cache_id, 'cache_filter')) {
      return $cached->data;
    }
  }

  // Convert all Windows and Mac newlines to a single newline, so filters only
  // need to deal with one possibility.
  $text = str_replace(array("\r\n", "\r"), "\n", $text);

  // Get a complete list of filters, ordered properly.
  $filters = filter_list_format($format->format);
  $filter_info = filter_get_filters();

  // Give filters the chance to escape HTML-like data such as code or formulas.
  foreach ($filters as $name => $filter) {
    if ($filter->status && isset($filter_info[$name]['prepare callback']) && function_exists($filter_info[$name]['prepare callback'])) {
      $function = $filter_info[$name]['prepare callback'];
      $text = $function($text, $filter, $format, $langcode, $cache, $cache_id);
    }
  }

  // Perform filtering.
  foreach ($filters as $name => $filter) {
    if ($filter->status && isset($filter_info[$name]['process callback']) && function_exists($filter_info[$name]['process callback'])) {
      $function = $filter_info[$name]['process callback'];
      $text = $function($text, $filter, $format, $langcode, $cache, $cache_id);
    }
  }

  // Store in cache with a minimum expiration time of 1 day.
  if ($cache) {
    cache_set($cache_id, $text, 'cache_filter', REQUEST_TIME + (60 * 60 * 24));
  }

  return $text;
}

/**
 * Expands an element into a base element with text format selector attached.
 *
 * The form element will be expanded into two separate form elements, one
 * holding the original element, and the other holding the text format selector:
 * - value: Holds the original element, having its #type changed to the value of
 *   #base_type or 'textarea' by default.
 * - format: Holds the text format fieldset and the text format selection, using
 *   the text format id specified in #format or the user's default format by
 *   default, if NULL.
 *
 * The resulting value for the element will be an array holding the value and the
 * format.  For example, the value for the body element will be:
 * @code
 *   $form_state['values']['body']['value'] = 'foo';
 *   $form_state['values']['body']['format'] = 'foo';
 * @endcode
 *
 * @param $element
 *   The form element to process. Properties used:
 *   - #base_type: The form element #type to use for the 'value' element.
 *     'textarea' by default.
 *   - #format: (optional) The text format id to preselect. If 0, NULL, or not
 *     set, the default format for the current user will be used.
 *
 * @return
 *   The expanded element.
 */
function filter_process_format($element) {
  global $user;

  // Ensure that children appear as subkeys of this element.
  $element['#tree'] = TRUE;
  $blacklist = array(
    // Make form_builder() regenerate child properties.
    '#parents',
    '#id',
    '#name',
    // Do not copy this #process function to prevent form_builder() from
    // recursing infinitely.
    '#process',
    // Description is handled by theme_text_format_wrapper().
    '#description',
    // Ensure proper ordering of children.
    '#weight',
    // Properties already processed for the parent element.
    '#prefix',
    '#suffix',
    '#attached',
    '#processed',
    '#theme_wrappers',
  );
  // Move this element into sub-element 'value'.
  unset($element['value']);
  foreach (element_properties($element) as $key) {
    if (!in_array($key, $blacklist)) {
      $element['value'][$key] = $element[$key];
    }
  }

  $element['value']['#type'] = $element['#base_type'];
  $element['value'] += element_info($element['#base_type']);

  // Turn original element into a text format wrapper.
  $path = drupal_get_path('module', 'filter');
  $element['#attached']['js'][] = $path . '/filter.js';
  $element['#attached']['css'][] = $path . '/filter.css';

  // Setup child container for the text format widget.
  $element['format'] = array(
    '#type' => 'fieldset',
    '#attributes' => array('class' => array('filter-wrapper')),
  );

  // Prepare text format guidelines.
  $element['format']['guidelines'] = array(
    '#type' => 'container',
    '#attributes' => array('class' => array('filter-guidelines')),
    '#weight' => 20,
  );
  // Get a list of formats that the current user has access to.
  $formats = filter_formats($user);
  foreach ($formats as $format) {
    $options[$format->format] = $format->name;
    $element['format']['guidelines'][$format->format] = array(
      '#theme' => 'filter_guidelines',
      '#format' => $format,
    );
  }

  // Use the default format for this user if none was selected.
  if (empty($element['#format'])) {
    $element['#format'] = filter_default_format($user);
  }
  $element['format']['format'] = array(
    '#type' => 'select',
    '#title' => t('Text format'),
    '#options' => $options,
    '#default_value' => $element['#format'],
    '#access' => count($formats) > 1,
    '#weight' => 10,
    '#attributes' => array('class' => array('filter-list')),
    '#parents' => array_merge($element['#parents'], array('format')),
  );

  $element['format']['help'] = array(
    '#type' => 'container',
    '#theme' => 'filter_tips_more_info',
    '#attributes' => array('class' => array('filter-help')),
    '#weight' => 0,
  );

  // Lastly, disallow editing of this field if the user is not allowed to use
  // the stored and preselected text format. But only, if that format actually
  // exists.
  $all_formats = filter_formats();
  if (!isset($formats[$element['#format']]) && isset($all_formats[$element['#format']])) {
    // Overload default values into #value to make them unalterable.
    $element['value']['#value'] = $element['value']['#default_value'];
    $element['format']['format']['#value'] = $element['format']['format']['#default_value'];

    // Prepend #pre_render callback to replace field value with user notice
    // prior to rendering.
    if (!isset($element['value']['#pre_render'])) {
      $element['value']['#pre_render'] = array();
    }
    array_unshift($element['value']['#pre_render'], 'filter_form_access_denied');

    // Cosmetic adjustments.
    if (isset($element['value']['#rows'])) {
      $element['value']['#rows'] = 3;
    }
    $element['value']['#disabled'] = TRUE;
    $element['value']['#resizable'] = FALSE;

    // Hide the text format selector and any other child element (such as text
    // field's summary).
    foreach (element_children($element) as $key) {
      if ($key != 'value') {
        $element[$key]['#access'] = FALSE;
      }
    }
  }

  return $element;
}

/**
 * #pre_render callback for #type 'text_format' to hide field value from prying eyes.
 *
 * To not break form processing and previews if a user does not have access to a
 * stored text format, the expanded form elements in filter_process_format() are
 * forced to take over the stored #default_values for 'value' and 'format'.
 * However, to prevent the unfiltered, original #value from being displayed to
 * the user, we replace it with a friendly notice here.
 *
 * @see filter_process_format()
 */
function filter_form_access_denied($element) {
  $element['#value'] = t('This field has been disabled because you do not have sufficient permissions to edit it.');
  return $element;
}

/**
 * Returns HTML for a text format-enabled form element.
 *
 * @param $variables
 *   An associative array containing:
 *   - element: A render element containing #children and #description.
 *
 * @ingroup themeable
 */
function theme_text_format_wrapper($variables) {
  $element = $variables['element'];
  $output = '<div class="text-format-wrapper">';
  $output .= $element['#children'];
  if (!empty($element['#description'])) {
    $output .= '<div class="description">' . $element['#description'] . '</div>';
  }
  $output .= "</div>\n";

  return $output;
}

/**
 * Checks if a user has access to a particular text format.
 *
 * @param $format
 *   An object representing the text format.
 * @param $account
 *   (optional) The user account to check access for; if omitted, the currently
 *   logged-in user is used.
 *
 * @return
 *   Boolean TRUE if the user is allowed to access the given format.
 */
function filter_access($format, $account = NULL) {
  global $user;
  if (!isset($account)) {
    $account = $user;
  }
  // Handle special cases up front. All users have access to the fallback
  // format.
  if ($format->format == filter_fallback_format()) {
    return TRUE;
  }
  // Check the permission if one exists; otherwise, we have a non-existent
  // format so we return FALSE.
  $permission = filter_permission_name($format);
  return !empty($permission) && user_access($permission, $account);
}

/**
 * Helper function for fetching filter tips.
 */
function _filter_tips($format_id, $long = FALSE) {
  global $user;

  $formats = filter_formats($user);
  $filter_info = filter_get_filters();

  $tips = array();

  // If only listing one format, extract it from the $formats array.
  if ($format_id != -1) {
    $formats = array($formats[$format_id]);
  }

  foreach ($formats as $format) {
    $filters = filter_list_format($format->format);
    $tips[$format->name] = array();
    foreach ($filters as $name => $filter) {
      if ($filter->status && isset($filter_info[$name]['tips callback']) && function_exists($filter_info[$name]['tips callback'])) {
        $tip = $filter_info[$name]['tips callback']($filter, $format, $long);
        $tips[$format->name][$name] = array('tip' => $tip, 'id' => $name);
      }
    }
  }

  return $tips;
}

/**
 * Parses an HTML snippet and returns it as a DOM object.
 *
 * This function loads the body part of a partial (X)HTML document
 * and returns a full DOMDocument object that represents this document.
 * You can use filter_dom_serialize() to serialize this DOMDocument
 * back to a XHTML snippet.
 *
 * @param $text
 *   The partial (X)HTML snippet to load. Invalid mark-up
 *   will be corrected on import.
 * @return
 *   A DOMDocument that represents the loaded (X)HTML snippet.
 */
function filter_dom_load($text) {
  // Ignore warnings during HTML soup loading.
  $dom_document = @DOMDocument::loadHTML('<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8" /></head><body>' . $text . '</body></html>');

  return $dom_document;
}

/**
 * Converts a DOM object back to an HTML snippet.
 *
 * The function serializes the body part of a DOMDocument
 * back to an XHTML snippet.
 *
 * The resulting XHTML snippet will be properly formatted
 * to be compatible with HTML user agents.
 *
 * @param $dom_document