theme.inc

<?php

/**
 * @file
 * The theme system, which controls the output of Drupal.
 *
 * The theme system allows for nearly all output of the Drupal system to be
 * customized by user themes.
 */

use Drupal\Core\Utility\ThemeRegistry;

/**
 * @defgroup content_flags Content markers
 * @{
 * Markers used by theme_mark() and node_mark() to designate content.
 * @see theme_mark(), node_mark()
 */

/**
 * Mark content as read.
 */
const MARK_READ = 0;

/**
 * Mark content as being new.
 */
const MARK_NEW = 1;

/**
 * Mark content as being updated.
 */
const MARK_UPDATED = 2;

/**
 * @} End of "defgroup content_flags".
 */

/**
 * Determines if a theme is available to use.
 *
 * @param $theme
 *   Either the name of a theme or a full theme object.
 *
 * @return
 *   Boolean TRUE if the theme is enabled or is the site administration theme;
 *   FALSE otherwise.
 */
function drupal_theme_access($theme) {
  if (is_object($theme)) {
    return !empty($theme->status);
  }
  else {
    $themes = list_themes();
    return !empty($themes[$theme]->status);
  }
}

/**
 * Initialize the theme system by loading the theme.
 */
function drupal_theme_initialize() {
  global $theme, $user, $theme_key;

  // If $theme is already set, assume the others are set, too, and do nothing
  if (isset($theme)) {
    return;
  }

  drupal_bootstrap(DRUPAL_BOOTSTRAP_DATABASE);
  $themes = list_themes();

  // Only select the user selected theme if it is available in the
  // list of themes that can be accessed.
  $theme = !empty($user->theme) && drupal_theme_access($user->theme) ? $user->theme : variable_get('theme_default', 'stark');

  // Allow modules to override the theme. Validation has already been performed
  // inside menu_get_custom_theme(), so we do not need to check it again here.
  $custom_theme = menu_get_custom_theme();
  $theme = !empty($custom_theme) ? $custom_theme : $theme;

  // Store the identifier for retrieving theme settings with.
  $theme_key = $theme;

  // Find all our ancestor themes and put them in an array.
  $base_theme = array();
  $ancestor = $theme;
  while ($ancestor && isset($themes[$ancestor]->base_theme)) {
    $ancestor = $themes[$ancestor]->base_theme;
    $base_theme[] = $themes[$ancestor];
  }
  _drupal_theme_initialize($themes[$theme], array_reverse($base_theme));

  // Themes can have alter functions, so reset the drupal_alter() cache.
  drupal_static_reset('drupal_alter');

  // Provide the page with information about the theme that's used, so that a
  // later Ajax request can be rendered using the same theme.
  // @see ajax_base_page_theme()
  $setting['ajaxPageState'] = array(
    'theme' => $theme_key,
    'theme_token' => drupal_get_token($theme_key),
  );
  drupal_add_js($setting, 'setting');
}

/**
 * Initialize the theme system given already loaded information. This
 * function is useful to initialize a theme when no database is present.
 *
 * @param $theme
 *   An object with the following information:
 *     filename
 *       The .info file for this theme. The 'path' to
 *       the theme will be in this file's directory. (Required)
 *     owner
 *       The path to the .theme file or the .engine file to load for
 *       the theme. (Required)
 *     stylesheet
 *       The primary stylesheet for the theme. (Optional)
 *     engine
 *       The name of theme engine to use. (Optional)
 * @param $base_theme
 *    An optional array of objects that represent the 'base theme' if the
 *    theme is meant to be derivative of another theme. It requires
 *    the same information as the $theme object. It should be in
 *    'oldest first' order, meaning the top level of the chain will
 *    be first.
 * @param $registry_callback
 *   The callback to invoke to set the theme registry.
 */
function _drupal_theme_initialize($theme, $base_theme = array(), $registry_callback = '_theme_load_registry') {
  global $theme_info, $base_theme_info, $theme_engine, $theme_path;
  $theme_info = $theme;
  $base_theme_info = $base_theme;

  $theme_path = dirname($theme->filename);

  // Prepare stylesheets from this theme as well as all ancestor themes.
  // We work it this way so that we can have child themes override parent
  // theme stylesheets easily.
  $final_stylesheets = array();

  // Grab stylesheets from base theme
  foreach ($base_theme as $base) {
    if (!empty($base->stylesheets)) {
      foreach ($base->stylesheets as $media => $stylesheets) {
        foreach ($stylesheets as $name => $stylesheet) {
          $final_stylesheets[$media][$name] = $stylesheet;
        }
      }
    }
  }

  // Add stylesheets used by this theme.
  if (!empty($theme->stylesheets)) {
    foreach ($theme->stylesheets as $media => $stylesheets) {
      foreach ($stylesheets as $name => $stylesheet) {
        $final_stylesheets[$media][$name] = $stylesheet;
      }
    }
  }

  // And now add the stylesheets properly
  foreach ($final_stylesheets as $media => $stylesheets) {
    foreach ($stylesheets as $stylesheet) {
      drupal_add_css($stylesheet, array('group' => CSS_THEME, 'every_page' => TRUE, 'media' => $media));
    }
  }

  // Do basically the same as the above for scripts
  $final_scripts = array();

  // Grab scripts from base theme
  foreach ($base_theme as $base) {
    if (!empty($base->scripts)) {
      foreach ($base->scripts as $name => $script) {
        $final_scripts[$name] = $script;
      }
    }
  }

  // Add scripts used by this theme.
  if (!empty($theme->scripts)) {
    foreach ($theme->scripts as $name => $script) {
      $final_scripts[$name] = $script;
    }
  }

  // Add scripts used by this theme.
  foreach ($final_scripts as $script) {
    drupal_add_js($script, array('group' => JS_THEME, 'every_page' => TRUE));
  }

  $theme_engine = NULL;

  // Initialize the theme.
  if (isset($theme->engine)) {
    // Include the engine.
    include_once DRUPAL_ROOT . '/' . $theme->owner;

    $theme_engine = $theme->engine;
    if (function_exists($theme_engine . '_init')) {
      foreach ($base_theme as $base) {
        call_user_func($theme_engine . '_init', $base);
      }
      call_user_func($theme_engine . '_init', $theme);
    }
  }
  else {
    // include non-engine theme files
    foreach ($base_theme as $base) {
      // Include the theme file or the engine.
      if (!empty($base->owner)) {
        include_once DRUPAL_ROOT . '/' . $base->owner;
      }
    }
    // and our theme gets one too.
    if (!empty($theme->owner)) {
      include_once DRUPAL_ROOT . '/' . $theme->owner;
    }
  }

  if (isset($registry_callback)) {
    _theme_registry_callback($registry_callback, array($theme, $base_theme, $theme_engine));
  }
}

/**
 * Get the theme registry.
 *
 * @param bool $complete
 *   Optional boolean to indicate whether to return the complete theme registry
 *   array or an instance of the Drupal\Core\Utility\ThemeRegistry class.
 *   If TRUE, the complete theme registry array will be returned. This is useful
 *   if you want to foreach over the whole registry, use array_* functions or
 *   inspect it in a debugger. If FALSE, an instance of the
 *   Drupal\Core\Utility\ThemeRegistry class will be returned, this provides an
 *   ArrayObject which allows it to be accessed with array syntax and isset(),
 *   and should be more lightweight than the full registry. Defaults to TRUE.
 *
 * @return
 *   The complete theme registry array, or an instance of the
 *   Drupal\Core\Utility\ThemeRegistry class.
 */
function theme_get_registry($complete = TRUE) {
  // Use the advanced drupal_static() pattern, since this is called very often.
  static $drupal_static_fast;
  if (!isset($drupal_static_fast)) {
    $drupal_static_fast['registry'] = &drupal_static('theme_get_registry');
  }
  $theme_registry = &$drupal_static_fast['registry'];

  // Initialize the theme, if this is called early in the bootstrap, or after
  // static variables have been reset.
  if (!is_array($theme_registry)) {
    drupal_theme_initialize();
    $theme_registry = array();
  }

  $key = (int) $complete;

  if (!isset($theme_registry[$key])) {
    list($callback, $arguments) = _theme_registry_callback();
    if (!$complete) {
      $arguments[] = FALSE;
    }
    $theme_registry[$key] = call_user_func_array($callback, $arguments);
  }

  return $theme_registry[$key];
}

/**
 * Set the callback that will be used by theme_get_registry() to fetch the registry.
 *
 * @param $callback
 *   The name of the callback function.
 * @param $arguments
 *   The arguments to pass to the function.
 */
function _theme_registry_callback($callback = NULL, array $arguments = array()) {
  static $stored;
  if (isset($callback)) {
    $stored = array($callback, $arguments);
  }
  return $stored;
}

/**
 * Get the theme_registry cache; if it doesn't exist, build it.
 *
 * @param $theme
 *   The loaded $theme object as returned by list_themes().
 * @param $base_theme
 *   An array of loaded $theme objects representing the ancestor themes in
 *   oldest first order.
 * @param $theme_engine
 *   The name of the theme engine.
 * @param $complete
 *   Whether to load the complete theme registry or an instance of the
 *   Drupal\Core\Utility\ThemeRegistry class.
 *
 * @return
 *   The theme registry array, or an instance of the
 *   Drupal\Core\Utility\ThemeRegistry class.
 */
function _theme_load_registry($theme, $base_theme = NULL, $theme_engine = NULL, $complete = TRUE) {
  if ($complete) {
    // Check the theme registry cache; if it exists, use it.
    $cached = cache()->get("theme_registry:$theme->name");
    if (isset($cached->data)) {
      $registry = $cached->data;
    }
    else {
      // If not, build one and cache it.
      $registry = _theme_build_registry($theme, $base_theme, $theme_engine);
      // Only persist this registry if all modules are loaded. This assures a
      // complete set of theme hooks.
      if (module_load_all(NULL)) {
        _theme_save_registry($theme, $registry);
      }
    }
    return $registry;
  }
  else {
    return new ThemeRegistry('theme_registry:runtime:' . $theme->name, 'cache');
  }
}

/**
 * Write the theme_registry cache into the database.
 */
function _theme_save_registry($theme, $registry) {
  cache()->set("theme_registry:$theme->name", $registry);
}

/**
 * Force the system to rebuild the theme registry; this should be called
 * when modules are added to the system, or when a dynamic system needs
 * to add more theme hooks.
 */
function drupal_theme_rebuild() {
  drupal_static_reset('theme_get_registry');
  cache()->deletePrefix('theme_registry');
}

/**
 * Process a single implementation of hook_theme().
 *
 * @param $cache
 *   The theme registry that will eventually be cached; It is an associative
 *   array keyed by theme hooks, whose values are associative arrays describing
 *   the hook:
 *   - 'type': The passed-in $type.
 *   - 'theme path': The passed-in $path.
 *   - 'function': The name of the function generating output for this theme
 *     hook. Either defined explicitly in hook_theme() or, if neither 'function'
 *     nor 'template' is defined, then the default theme function name is used.
 *     The default theme function name is the theme hook prefixed by either
 *     'theme_' for modules or '$name_' for everything else. If 'function' is
 *     defined, 'template' is not used.
 *   - 'template': The filename of the template generating output for this
 *     theme hook. The template is in the directory defined by the 'path' key of
 *     hook_theme() or defaults to $path.
 *   - 'variables': The variables for this theme hook as defined in
 *     hook_theme(). If there is more than one implementation and 'variables' is
 *     not specified in a later one, then the previous definition is kept.
 *   - 'render element': The renderable element for this theme hook as defined
 *     in hook_theme(). If there is more than one implementation and
 *     'render element' is not specified in a later one, then the previous
 *     definition is kept.
 *   - 'preprocess functions': See theme() for detailed documentation.
 *   - 'process functions': See theme() for detailed documentation.
 * @param $name
 *   The name of the module, theme engine, base theme engine, theme or base
 *   theme implementing hook_theme().
 * @param $type
 *   One of 'module', 'theme_engine', 'base_theme_engine', 'theme', or
 *   'base_theme'. Unlike regular hooks that can only be implemented by modules,
 *   each of these can implement hook_theme(). _theme_process_registry() is
 *   called in aforementioned order and new entries override older ones. For
 *   example, if a theme hook is both defined by a module and a theme, then the
 *   definition in the theme will be used.
 * @param $theme
 *   The loaded $theme object as returned from list_themes().
 * @param $path
 *   The directory where $name is. For example, modules/system or
 *   themes/bartik.
 *
 * @see theme()
 * @see _theme_process_registry()
 * @see hook_theme()
 * @see list_themes()
 */
function _theme_process_registry(&$cache, $name, $type, $theme, $path) {
  $result = array();

  // Processor functions work in two distinct phases with the process
  // functions always being executed after the preprocess functions.
  $variable_process_phases = array(
    'preprocess functions' => 'preprocess',
    'process functions'    => 'process',
  );

  $hook_defaults = array(
    'variables' => TRUE,
    'render element' => TRUE,
    'pattern' => TRUE,
    'base hook' => TRUE,
  );

  // Invoke the hook_theme() implementation, process what is returned, and
  // merge it into $cache.
  $function = $name . '_theme';
  if (function_exists($function)) {
    $result = $function($cache, $type, $theme, $path);
    foreach ($result as $hook => $info) {
      // When a theme or engine overrides a module's theme function
      // $result[$hook] will only contain key/value pairs for information being
      // overridden.  Pull the rest of the information from what was defined by
      // an earlier hook.

      // Fill in the type and path of the module, theme, or engine that
      // implements this theme function.
      $result[$hook]['type'] = $type;
      $result[$hook]['theme path'] = $path;

      // If function and file are omitted, default to standard naming
      // conventions.
      if (!isset($info['template']) && !isset($info['function'])) {
        $result[$hook]['function'] = ($type == 'module' ? 'theme_' : $name . '_') . $hook;
      }

      if (isset($cache[$hook]['includes'])) {
        $result[$hook]['includes'] = $cache[$hook]['includes'];
      }

      // If the theme implementation defines a file, then also use the path
      // that it defined. Otherwise use the default path. This allows
      // system.module to declare theme functions on behalf of core .include
      // files.
      if (isset($info['file'])) {
        $include_file = isset($info['path']) ? $info['path'] : $path;
        $include_file .= '/' . $info['file'];
        include_once DRUPAL_ROOT . '/' . $include_file;
        $result[$hook]['includes'][] = $include_file;
      }

      // If the default keys are not set, use the default values registered
      // by the module.
      if (isset($cache[$hook])) {
        $result[$hook] += array_intersect_key($cache[$hook], $hook_defaults);
      }

      // The following apply only to theming hooks implemented as templates.
      if (isset($info['template'])) {
        // Prepend the current theming path when none is set.
        if (!isset($info['path'])) {
          $result[$hook]['template'] = $path . '/' . $info['template'];
        }
      }

      // Allow variable processors for all theming hooks, whether the hook is
      // implemented as a template or as a function.
      foreach ($variable_process_phases as $phase_key => $phase) {
        // Check for existing variable processors. Ensure arrayness.
        if (!isset($info[$phase_key]) || !is_array($info[$phase_key])) {
          $info[$phase_key] = array();
          $prefixes = array();
          if ($type == 'module') {
            // Default variable processor prefix.
            $prefixes[] = 'template';
            // Add all modules so they can intervene with their own variable
            // processors. This allows them to provide variable processors even
            // if they are not the owner of the current hook.
            $prefixes += module_list();
          }
          elseif ($type == 'theme_engine' || $type == 'base_theme_engine') {
            // Theme engines get an extra set that come before the normally
            // named variable processors.
            $prefixes[] = $name . '_engine';
            // The theme engine registers on behalf of the theme using the
            // theme's name.
            $prefixes[] = $theme;
          }
          else {
            // This applies when the theme manually registers their own variable
            // processors.
            $prefixes[] = $name;
          }
          foreach ($prefixes as $prefix) {
            // Only use non-hook-specific variable processors for theming hooks
            // implemented as templates. See theme().
            if (isset($info['template']) && function_exists($prefix . '_' . $phase)) {
              $info[$phase_key][] = $prefix . '_' . $phase;
            }
            if (function_exists($prefix . '_' . $phase . '_' . $hook)) {
              $info[$phase_key][] = $prefix . '_' . $phase . '_' . $hook;
            }
          }
        }
        // Check for the override flag and prevent the cached variable
        // processors from being used. This allows themes or theme engines to
        // remove variable processors set earlier in the registry build.
        if (!empty($info['override ' . $phase_key])) {
          // Flag not needed inside the registry.
          unset($result[$hook]['override ' . $phase_key]);
        }
        elseif (isset($cache[$hook][$phase_key]) && is_array($cache[$hook][$phase_key])) {
          $info[$phase_key] = array_merge($cache[$hook][$phase_key], $info[$phase_key]);
        }
        $result[$hook][$phase_key] = $info[$phase_key];
      }
    }

    // Merge the newly created theme hooks into the existing cache.
    $cache = $result + $cache;
  }

  // Let themes have variable processors even if they didn't register a template.
  if ($type == 'theme' || $type == 'base_theme') {
    foreach ($cache as $hook => $info) {
      // Check only if not registered by the theme or engine.
      if (empty($result[$hook])) {
        foreach ($variable_process_phases as $phase_key => $phase) {
          if (!isset($info[$phase_key])) {
            $cache[$hook][$phase_key] = array();
          }
          // Only use non-hook-specific variable processors for theming hooks
          // implemented as templates. See theme().
          if (isset($info['template']) && function_exists($name . '_' . $phase)) {
            $cache[$hook][$phase_key][] = $name . '_' . $phase;
          }
          if (function_exists($name . '_' . $phase . '_' . $hook)) {
            $cache[$hook][$phase_key][] = $name . '_' . $phase . '_' . $hook;
            $cache[$hook]['theme path'] = $path;
          }
          // Ensure uniqueness.
          $cache[$hook][$phase_key] = array_unique($cache[$hook][$phase_key]);
        }
      }
    }
  }
}

/**
 * Build the theme registry cache.
 *
 * @param $theme
 *   The loaded $theme object as returned by list_themes().
 * @param $base_theme
 *   An array of loaded $theme objects representing the ancestor themes in
 *   oldest first order.
 * @param $theme_engine
 *   The name of the theme engine.
 */
function _theme_build_registry($theme, $base_theme, $theme_engine) {
  $cache = array();
  // First, process the theme hooks advertised by modules. This will
  // serve as the basic registry. Since the list of enabled modules is the same
  // regardless of the theme used, this is cached in its own entry to save
  // building it for every theme.
  if ($cached = cache()->get('theme_registry:build:modules')) {
    $cache = $cached->data;
  }
  else {
    foreach (module_implements('theme') as $module) {
      _theme_process_registry($cache, $module, 'module', $module, drupal_get_path('module', $module));
    }
    // Only cache this registry if all modules are loaded.
    if (module_load_all(NULL)) {
      cache()->set('theme_registry:build:modules', $cache);
    }
  }

  // Process each base theme.
  foreach ($base_theme as $base) {
    // If the base theme uses a theme engine, process its hooks.
    $base_path = dirname($base->filename);
    if ($theme_engine) {
      _theme_process_registry($cache, $theme_engine, 'base_theme_engine', $base->name, $base_path);
    }
    _theme_process_registry($cache, $base->name, 'base_theme', $base->name, $base_path);
  }

  // And then the same thing, but for the theme.
  if ($theme_engine) {
    _theme_process_registry($cache, $theme_engine, 'theme_engine', $theme->name, dirname($theme->filename));
  }

  // Finally, hooks provided by the theme itself.
  _theme_process_registry($cache, $theme->name, 'theme', $theme->name, dirname($theme->filename));

  // Let modules alter the registry.
  drupal_alter('theme_registry', $cache);

  // Optimize the registry to not have empty arrays for functions.
  foreach ($cache as $hook => $info) {
    foreach (array('preprocess functions', 'process functions') as $phase) {
      if (empty($info[$phase])) {
        unset($cache[$hook][$phase]);
      }
    }
  }
  return $cache;
}

/**
 * Return a list of all currently available themes.
 *
 * Retrieved from the database, if available and the site is not in maintenance
 * mode; otherwise compiled freshly from the filesystem.
 *
 * @param $refresh
 *   Whether to reload the list of themes from the database. Defaults to FALSE.
 *
 * @return
 *   An associative array of the currently available themes. The keys are the
 *   themes' machine names and the values are objects having the following
 *   properties:
 *   - filename: The filepath and name of the .info file.
 *   - name: The machine name of the theme.
 *   - status: 1 for enabled, 0 for disabled themes.
 *   - info: The contents of the .info file.
 *   - stylesheets: A two dimensional array, using the first key for the
 *     media attribute (e.g. 'all'), the second for the name of the file
 *     (e.g. style.css). The value is a complete filepath (e.g.
 *     themes/bartik/style.css). Not set if no stylesheets are defined in the
 *     .info file.
 *   - scripts: An associative array of JavaScripts, using the filename as key
 *     and the complete filepath as value. Not set if no scripts are defined in
 *     the .info file.
 *   - prefix: The base theme engine prefix.
 *   - engine: The machine name of the theme engine.
 *   - base_theme: If this is a sub-theme, the machine name of the base theme
 *     defined in the .info file. Otherwise, the element is not set.
 *   - base_themes: If this is a sub-theme, an associative array of the
 *     base-theme ancestors of this theme, starting with this theme's base
 *     theme, then the base theme's own base theme, etc. Each entry has an
 *     array key equal to the theme's machine name, and a value equal to the
 *     human-readable theme name; if a theme with matching machine name does
 *     not exist in the system, the value will instead be NULL (and since the
 *     system would not know whether that theme itself has a base theme, that
 *     will end the array of base themes). This is not set if the theme is not
 *     a sub-theme.
 *   - sub_themes: An associative array of themes on the system that are
 *     either direct sub-themes (that is, they declare this theme to be
 *     their base theme), direct sub-themes of sub-themes, etc. The keys are
 *     the themes' machine names, and the values are the themes' human-readable
 *     names. This element is not set if there are no themes on the system that
 *     declare this theme as their base theme. 
*/
function list_themes($refresh = FALSE) {
  $list = &drupal_static(__FUNCTION__, array());

  if ($refresh) {
    $list = array();
    system_list_reset();
  }

  if (empty($list)) {
    $list = array();
    $themes = array();
    // Extract from the database only when it is available.
    // Also check that the site is not in the middle of an install or update.
    if (!defined('MAINTENANCE_MODE')) {
      try {
        $themes = system_list('theme');
      }
      catch (Exception $e) {
        // If the database is not available, rebuild the theme data.
        $themes = _system_rebuild_theme_data();
      }
    }
    else {
      // Scan the installation when the database should not be read.
      $themes = _system_rebuild_theme_data();
    }

    foreach ($themes as $theme) {
      foreach ($theme->info['stylesheets'] as $media => $stylesheets) {
        foreach ($stylesheets as $stylesheet => $path) {
          $theme->stylesheets[$media][$stylesheet] = $path;
        }
      }
      foreach ($theme->info['scripts'] as $script => $path) {
        $theme->scripts[$script] = $path;
      }
      if (isset($theme->info['engine'])) {
        $theme->engine = $theme->info['engine'];
      }
      if (isset($theme->info['base theme'])) {
        $theme->base_theme = $theme->info['base theme'];
      }
      // Status is normally retrieved from the database. Add zero values when
      // read from the installation directory to prevent notices.
      if (!isset($theme->status)) {
        $theme->status = 0;
      }
      $list[$theme->name] = $theme;
    }
  }

  return $list;
}

/**
 * Find all the base themes for the specified theme.
 *
 * Themes can inherit templates and function implementations from earlier themes.
 *
 * @param $themes
 *   An array of available themes.
 * @param $key
 *   The name of the theme whose base we are looking for.
 * @param $used_keys
 *   A recursion parameter preventing endless loops.
 * @return
 *   Returns an array of all of the theme's ancestors; the first element's value
 *   will be NULL if an error occurred.
 */
function drupal_find_base_themes($themes, $key, $used_keys = array()) {
  $base_key = $themes[$key]->info['base theme'];
  // Does the base theme exist?
  if (!isset($themes[$base_key])) {
    return array($base_key => NULL);
  }

  $current_base_theme = array($base_key => $themes[$base_key]->info['name']);

  // Is the base theme itself a child of another theme?
  if (isset($themes[$base_key]->info['base theme'])) {
    // Do we already know the base themes of this theme?
    if (isset($themes[$base_key]->base_themes)) {
      return $themes[$base_key]->base_themes + $current_base_theme;
    }
    // Prevent loops.
    if (!empty($used_keys[$base_key])) {
      return array($base_key => NULL);
    }
    $used_keys[$base_key] = TRUE;
    return drupal_find_base_themes($themes, $base_key, $used_keys) + $current_base_theme;
  }
  // If we get here, then this is our parent theme.
  return $current_base_theme;
}

/**
 * Generates themed output.
 *
 * All requests for themed output must go through this function. It examines
 * the request and routes it to the appropriate
 * @link themeable theme function or template @endlink, by checking the theme
 * registry.
 *
 * Most commonly, the first argument to this function is the name of the theme
 * hook. For instance, to theme a taxonomy term, the theme hook name is
 * 'taxonomy_term'. Modules register theme hooks within a hook_theme()
 * implementation and provide a default implementation via a function named
 * theme_HOOK() (e.g., theme_taxonomy_term()) or via a template file named
 * according to the value of the 'template' key registered with the theme hook
 * (see hook_theme() for details). Default templates are implemented with the
 * PHPTemplate rendering engine and are named the same as the theme hook, with
 * underscores changed to hyphens, so for the 'taxonomy_term' theme hook, the
 * default template is 'taxonomy-term.tpl.php'.
 *
 * Themes may also register new theme hooks within a hook_theme()
 * implementation, but it is more common for themes to override default
 * implementations provided by modules than to register entirely new theme
 * hooks. Themes can override a default implementation by implementing a
 * function named THEME_HOOK() (for example, the 'bartik' theme overrides the
 * default implementation of the 'menu_tree' theme hook by implementing a
 * bartik_menu_tree() function), or by adding a template file within its folder
 * structure that follows the template naming structure used by the theme's
 * rendering engine (for example, since the Bartik theme uses the PHPTemplate
 * rendering engine, it overrides the default implementation of the 'page' theme
 * hook by containing a 'page.tpl.php' file within its folder structure).
 *
 * If the implementation is a template file, several functions are called
 * before the template file is invoked, to modify the $variables array. These
 * fall into the "preprocessing" phase and the "processing" phase, and are
 * executed (if they exist), in the following order (note that in the following
 * list, HOOK indicates the theme hook name, MODULE indicates a module name,
 * THEME indicates a theme name, and ENGINE indicates a theme engine name):
 * - template_preprocess(&$variables, $hook): Creates a default set of variables
 *   for all theme hooks with template implementations.
 * - template_preprocess_HOOK(&$variables): Should be implemented by the module
 *   that registers the theme hook, to set up default variables.
 * - MODULE_preprocess(&$variables, $hook): hook_preprocess() is invoked on all
 *   implementing modules.
 * - MODULE_preprocess_HOOK(&$variables): hook_preprocess_HOOK() is invoked on
 *   all implementing modules, so that modules that didn't define the theme hook
 *   can alter the variables.
 * - ENGINE_engine_preprocess(&$variables, $hook): Allows the theme engine to
 *   set necessary variables for all theme hooks with template implementations.
 * - ENGINE_engine_preprocess_HOOK(&$variables): Allows the theme engine to set
 *   necessary variables for the particular theme hook.
 * - THEME_preprocess(&$variables, $hook): Allows the theme to set necessary
 *   variables for all theme hooks with template implementations.
 * - THEME_preprocess_HOOK(&$variables): Allows the theme to set necessary
 *   variables specific to the particular theme hook.
 * - template_process(&$variables, $hook): Creates an additional set of default
 *   variables for all theme hooks with template implementations. The variables
 *   created in this function are derived from ones created by
 *   template_preprocess(), but potentially altered by the other preprocess
 *   functions listed above. For example, any preprocess function can add to or
 *   modify the $variables['attributes_array'] variable, and after all of them
 *   have finished executing, template_process() flattens it into a
 *   $variables['attributes'] string for convenient use by templates.
 * - template_process_HOOK(&$variables): Should be implemented by the module
 *   that registers the theme hook, if it needs to perform additional variable
 *   processing after all preprocess functions have finished.
 * - MODULE_process(&$variables, $hook): hook_process() is invoked on all
 *   implementing modules.
 * - MODULE_process_HOOK(&$variables): hook_process_HOOK() is invoked on
 *   on all implementing modules, so that modules that didn't define the theme
 *   hook can alter the variables.
 * - ENGINE_engine_process(&$variables, $hook): Allows the theme engine to
 *   process variables for all theme hooks with template implementations.
 * - ENGINE_engine_process_HOOK(&$variables): Allows the theme engine to process
 *   the variables specific to the theme hook.
 * - THEME_process(&$variables, $hook):  Allows the theme to process the
 *   variables for all theme hooks with template implementations.
 * - THEME_process_HOOK(&$variables):  Allows the theme to process the
 *   variables specific to the theme hook.
 *
 * If the implementation is a function, only the theme-hook-specific preprocess
 * and process functions (the ones ending in _HOOK) are called from the
 * list above. This is because theme hooks with function implementations
 * need to be fast, and calling the non-theme-hook-specific preprocess and
 * process functions for them would incur a noticeable performance penalty.
 *
 * There are two special variables that these preprocess and process functions
 * can set: 'theme_hook_suggestion' and 'theme_hook_suggestions'. These will be
 * merged together to form a list of 'suggested' alternate theme hooks to use,
 * in reverse order of priority. theme_hook_suggestion will always be a higher
 * priority than items in theme_hook_suggestions. theme() will use the
 * highest priority implementation that exists. If none exists, theme() will
 * use the implementation for the theme hook it was called with. These
 * suggestions are similar to and are used for similar reasons as calling
 * theme() with an array as the $hook parameter (see below). The difference
 * is whether the suggestions are determined by the code that calls theme() or
 * by a preprocess or process function.
 *
 * @param $hook
 *   The name of the theme hook to call. If the name contains a
 *   double-underscore ('__') and there isn't an implementation for the full
 *   name, the part before the '__' is checked. This allows a fallback to a more
 *   generic implementation. For example, if theme('links__node', ...) is
 *   called, but there is no implementation of that theme hook, then the 'links'
 *   implementation is used. This process is iterative, so if
 *   theme('links__contextual__node', ...) is called, theme() checks for the
 *   following implementations, and uses the first one that exists:
 *   - links__contextual__node
 *   - links__contextual
 *   - links
 *   This allows themes to create specific theme implementations for named
 *   objects and contexts of otherwise generic theme hooks. The $hook parameter
 *   may also be an array, in which case the first theme hook that has an
 *   implementation is used. This allows for the code that calls theme() to
 *   explicitly specify the fallback order in a situation where using the '__'
 *   convention is not desired or is insufficient.
 * @param $variables
 *   An associative array of variables to merge with defaults from the theme
 *   registry, pass to preprocess and process functions for modification, and
 *   finally, pass to the function or template implementing the theme hook.
 *   Alternatively, this can be a renderable array, in which case, its
 *   properties are mapped to variables expected by the theme hook
 *   implementations.
 *
 * @return
 *   An HTML string representing the themed output.
 *
 * @see themeable
 * @see hook_theme()
 * @see template_preprocess()
 * @see template_process()
 */
function theme($hook, $variables = array()) {
  // If called before all modules are loaded, we do not necessarily have a full
  // theme registry to work with, and therefore cannot process the theme
  // request properly. See also _theme_load_registry().
  if (!module_load_all(NULL) && !defined('MAINTENANCE_MODE')) {
    throw new Exception(t('theme() may not be called until all modules are loaded.'));
  }

  $hooks = theme_get_registry(FALSE);

  // If an array of hook candidates were passed, use the first one that has an
  // implementation.
  if (is_array($hook)) {
    foreach ($hook as $candidate) {
      if (isset($hooks[$candidate])) {
        break;
      }
    }
    $hook = $candidate;
  }

  // If there's no implementation, check for more generic fallbacks. If there's
  // still no implementation, log an error and return an empty string.
  if (!isset($hooks[$hook])) {
    // Iteratively strip everything after the last '__' delimiter, until an
    // implementation is found.
    while ($pos = strrpos($hook, '__')) {
      $hook = substr($hook, 0, $pos);
      if (isset($hooks[$hook])) {
        break;
      }
    }
    if (!isset($hooks[$hook])) {
      // Only log a message when not trying theme suggestions ($hook being an
      // array).
      if (!isset($candidate)) {
        watchdog('theme', 'Theme key "@key" not found.', array('@key' => $hook), WATCHDOG_WARNING);
      }
      return '';
    }
  }

  $info = $hooks[$hook];
  global $theme_path;
  $temp = $theme_path;
  // point path_to_theme() to the currently used theme path:
  $theme_path = $info['theme path'];

  // Include a file if the theme function or variable processor is held elsewhere.
  if (!empty($info['includes'])) {
    foreach ($info['includes'] as $include_file) {
      include_once DRUPAL_ROOT . '/' . $include_file;
    }
  }

  // If a renderable array is passed as $variables, then set $variables to
  // the arguments expected by the theme function.
  if (isset($variables['#theme']) || isset($variables['#theme_wrappers'])) {
    $element = $variables;
    $variables = array();
    if (isset($info['variables'])) {
      foreach (array_keys($info['variables']) as $name) {
        if (isset($element["#$name"])) {
          $variables[$name] = $element["#$name"];
        }
      }
    }
    else {
      $variables[$info['render element']] = $element;
    }
  }

  // Merge in argument defaults.
  if (!empty($info['variables'])) {
    $variables += $info['variables'];
  }
  elseif (!empty($info['render element'])) {
    $variables += array($info['render element'] => array());
  }

  // Invoke the variable processors, if any. The processors may specify
  // alternate suggestions for which hook's template/function to use. If the
  // hook is a suggestion of a base hook, invoke the variable processors of
  // the base hook, but retain the suggestion as a high priority suggestion to
  // be used unless overridden by a variable processor function.
  if (isset($info['base hook'])) {
    $base_hook = $info['base hook'];
    $base_hook_info = $hooks[$base_hook];
    // Include files required by the base hook, since its variable processors
    // might reside there.
    if (!empty($base_hook_info['includes'])) {
      foreach ($base_hook_info['includes'] as $include_file) {
        include_once DRUPAL_ROOT . '/' . $include_file;
      }
    }
    if (isset($base_hook_info['preprocess functions']) || isset($base_hook_info['process functions'])) {
      $variables['theme_hook_suggestion'] = $hook;
      $hook = $base_hook;
      $info = $base_hook_info;
    }
  }
  if (isset($info['preprocess functions']) || isset($info['process functions'])) {
    $variables['theme_hook_suggestions'] = array();
    foreach (array('preprocess functions', 'process functions') as $phase) {
      if (!empty($info[$phase])) {
        foreach ($info[$phase] as $processor_function) {
          if (function_exists($processor_function)) {
            // We don't want a poorly behaved process function changing $hook.
            $hook_clone = $hook;
            $processor_function($variables, $hook_clone);
          }
        }
      }
    }
    // If the preprocess/process functions specified hook suggestions, and the
    // suggestion exists in the theme registry, use it instead of the hook that
    // theme() was called with. This allows the preprocess/process step to
    // route to a more specific theme hook. For example, a function may call
    // theme('node', ...), but a preprocess function can add 'node__article' as
    // a suggestion, enabling a theme to have an alternate template file for
    // article nodes. Suggestions are checked in the following order: