theme.inc

<?php
// $Id$

/**
 * @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.
 *
 * @ingroup themeable
 */

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

/**
 * Mark content as read.
 */
define('MARK_READ', 0);

/**
 * Mark content as being new.
 */
define('MARK_NEW', 1);

/**
 * Mark content as being updated.
 */
define('MARK_UPDATED', 2);

/**
 * @} End of "Content markers".
 */

/**
 * Determines if a theme is available to use.
 *
 * @param $theme
 *   An object representing the theme to check.
 * @return
 *   Boolean TRUE if the theme is enabled or is the site administration theme;
 *   FALSE otherwise.
 */
function drupal_theme_access($theme) {
  $admin_theme = variable_get('admin_theme');
  return !empty($theme->status) || ($admin_theme && $theme->name == $admin_theme);
}

/**
 * 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) && isset($themes[$user->theme]) && drupal_theme_access($themes[$user->theme]) ? $user->theme : variable_get('theme_default', 'garland');

  // Allow modules to override the present theme... only select custom theme
  // if it is available in the list of themes that can be accessed.
  $custom_theme = menu_get_custom_theme();
  $theme = $custom_theme && isset($themes[$custom_theme]) && drupal_theme_access($themes[$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)) {
    $base_theme[] = $new_base_theme = $themes[$themes[$ancestor]->base_theme];
    $ancestor = $themes[$ancestor]->base_theme;
  }
  _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');
}

/**
 * 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('weight' => CSS_THEME, '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('weight' => JS_THEME));
  }

  $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 (function_exists($registry_callback)) {
    $registry_callback($theme, $base_theme, $theme_engine);
  }
}

/**
 * Get the theme registry.
 * @return
 *   The theme registry array if it has been stored in memory, NULL otherwise.
 */
function theme_get_registry() {
  return _theme_set_registry();
}

/**
 * Store the theme registry in memory.
 * @param $registry
 *   A registry array as returned by _theme_build_registry()
 * @return
 *   The theme registry array stored in memory
 */
function _theme_set_registry($registry = NULL) {
  static $theme_registry = NULL;

  if (isset($registry)) {
    $theme_registry = $registry;
  }

  return $theme_registry;
}

/**
 * Get the theme_registry cache from the database; 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 $theme objects representing the ancestor themes in
 *   oldest first order.
 * @param theme_engine
 *   The name of the theme engine.
 */
function _theme_load_registry($theme, $base_theme = NULL, $theme_engine = NULL) {
  // Check the theme registry cache; if it exists, use it.
  $cache = cache_get("theme_registry:$theme->name", 'cache');
  if (isset($cache->data)) {
    $registry = $cache->data;
    _theme_set_registry($registry);
  }
  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, $registry);
      _theme_set_registry($registry);
    }
  }
}

/**
 * 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() {
  cache_clear_all('theme_registry', 'cache', TRUE);
}

/**
 * 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.
 *   - 'theme paths': The paths where implementations of a theme hook can be
 *     found. Its definition is similarly inherited like 'variables'. Each time
 *     _theme_process_registry() is called for this theme hook, either the
 *     'path' key from hook_theme() (if defined) or $path is added.
 *   - '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 $name is. For example, modules/system or
 *   themes/garland.
 *
 * @see theme()
 * @see _theme_process_registry()
 * @see hook_theme()
 * @see list_themes()
 */
function _theme_process_registry(&$cache, $name, $type, $theme, $path) {
  $result = array();
  $function = $name . '_theme';

  // 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',
  );

  if (function_exists($function)) {
    $result = $function($cache, $type, $theme, $path);
    foreach ($result as $hook => $info) {
      $result[$hook]['type'] = $type;
      $result[$hook]['theme path'] = $path;
      // if function and file are left out, default to standard naming
      // conventions.
      if (!isset($info['template']) && !isset($info['function'])) {
        $result[$hook]['function'] = ($type == 'module' ? 'theme_' : $name . '_') . $hook;
      }
      // If a path is set in the info, use what was set. Otherwise use the
      // default path. This is mostly so system.module can declare theme
      // functions on behalf of core .include files.
      // All files are included to be safe. Conditionally included
      // files can prevent them from getting registered.
      if (isset($cache[$hook]['includes'])) {
        $result[$hook]['includes'] = $cache[$hook]['includes'];
      }
      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 'variables' have been defined previously, carry them forward.
      // This should happen if a theme overrides a Drupal defined theme
      // function, for example.
      if (!isset($info['variables']) && isset($cache[$hook]['variables'])) {
        $result[$hook]['variables'] = $cache[$hook]['variables'];
      }
      // Same for 'render element'.
      if (!isset($info['render element']) && isset($cache[$hook]['render element'])) {
        $result[$hook]['render element'] = $cache[$hook]['render element'];
      }

      // 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'];
        }

        // These are used for template naming suggestions. Theme implementations
        // can occur in multiple paths. Suggestions should follow.
        if (!isset($info['theme paths']) && isset($cache[$hook])) {
          $result[$hook]['theme paths'] = $cache[$hook]['theme paths'];
        }
        // Check for sub-directories.
        $result[$hook]['theme paths'][] = isset($info['path']) ? $info['path'] : $path;
      }

      // 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.
    $cache = array_merge($cache, $result);
  }

  // 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;
          }
          // Ensure uniqueness.
          $cache[$hook][$phase_key] = array_unique($cache[$hook][$phase_key]);
        }
      }
    }
  }
}

/**
 * Rebuild 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.
  foreach (module_implements('theme') as $module) {
    _theme_process_registry($cache, $module, 'module', $module, drupal_get_path('module', $module));
  }

  // 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 $refresh
 *   Whether to reload the list of themes from the database. Defaults to FALSE.
 * @return
 *   An associative array of the currently available themes. The keys are the
 *   names of the themes and the values are objects having the following
 *   properties:
 *   - 'filename': The name of the .info file.
 *   - 'name': The name of the theme.
 *   - 'status': 1 for enabled, 0 for disabled themes.
 *   - 'info': The contents of the .info file.
 *   - 'stylesheets': A two dimensional array, using the first key for the
 *     'media' attribute (e.g. 'all'), the second for the name of the file
 *     (e.g. style.css). The value is a complete filepath
 *     (e.g. themes/garland/style.css).
 *   - 'scripts': An associative array of JavaScripts, using the filename as key
 *     and the complete filepath as value.
 *   - 'engine': The name of the theme engine.
 *   - 'base theme': The name of the base theme.
 */
function list_themes($refresh = FALSE) {
  static $list = array();

  if ($refresh) {
    $list = array();
    drupal_static_reset('system_list');
  }

  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 (db_is_active() && !defined('MAINTENANCE_MODE')) {
      foreach (system_list('theme') as $theme) {
        if (file_exists($theme->filename)) {
          $theme->info = unserialize($theme->info);
          $themes[] = $theme;
        }
      }
    }
    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 => $path) {
        if (file_exists($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;
}

/**
 * Generate the themed output.
 *
 * All requests for theme hooks must go through this function. It examines
 * the request and routes it to the appropriate theme function. The theme
 * registry is checked to determine which implementation to use, which may
 * be a function or a template.
 *
 * If the implementation is a template, the following functions may be used to
 * modify the $variables array. They are processed in two distinct phases;
 * "preprocess" and "process" functions. The order listed here is the order in
 * which they execute.
 *
 * - template_preprocess(&$variables)
 *   This sets a default set of variables for all template implementations.
 *
 * - template_preprocess_HOOK(&$variables)
 *   This is the first preprocessor called specific to the hook; it should be
 *   implemented by the module that registers it.
 *
 * - MODULE_preprocess(&$variables)
 *   This will be called for all templates; it should only be used if there
 *   is a real need. It's purpose is similar to template_preprocess().
 *
 * - MODULE_preprocess_HOOK(&$variables)
 *   This is for modules that want to alter or provide extra variables for
 *   theming hooks not registered to itself. For example, if a module named
 *   "foo" wanted to alter the $classes_array variable for the hook "node" a
 *   preprocess function of foo_preprocess_node() can be created to intercept
 *   and alter the variable.
 *
 * - ENGINE_engine_preprocess(&$variables)
 *   This function should only be implemented by theme engines and exists
 *   so that it can set necessary variables for all hooks.
 *
 * - ENGINE_engine_preprocess_HOOK(&$variables)
 *   This is the same as the previous function, but it is called for a single
 *   theming hook.
 *
 * - THEME_preprocess(&$variables)
 *   This is for themes that want to alter or provide extra variables. For
 *   example, if a theme named "foo" wanted to alter the $classes_array variable
 *   for the hook "node" a preprocess function of foo_preprocess_node() can be
 *   created to intercept and alter the variable.
 *
 * - THEME_preprocess_HOOK(&$variables)
 *   The same applies from the previous function, but it is called for a
 *   specific hook.
 *
 * - template_process(&$variables)
 *   This sets a default set of variables for all template implementations.
 *
 * - template_process_HOOK(&$variables)
 *   This is the first processor called specific to the hook; it should be
 *   implemented by the module that registers it.
 *
 * - MODULE_process(&$variables)
 *   This will be called for all templates; it should only be used if there
 *   is a real need. It's purpose is similar to template_process().
 *
 * - MODULE_process_HOOK(&$variables)
 *   This is for modules that want to alter or provide extra variables for
 *   theming hooks not registered to itself. For example, if a module named
 *   "foo" wanted to alter the $classes_array variable for the hook "node" a
 *   process function of foo_process_node() can be created to intercept
 *   and alter the variable.
 *
 * - ENGINE_engine_process(&$variables)
 *   This function should only be implemented by theme engines and exists
 *   so that it can set necessary variables for all hooks.
 *
 * - ENGINE_engine_process_HOOK(&$variables)
 *   This is the same as the previous function, but it is called for a single
 *   theming hook.
 *
 * - ENGINE_process(&$variables)
 *   This is meant to be used by themes that utilize a theme engine. It is
 *   provided so that the processor is not locked into a specific theme.
 *   This makes it easy to share and transport code but theme authors must be
 *   careful to prevent fatal re-declaration errors when using sub-themes that
 *   have their own processor named exactly the same as its base theme. In
 *   the default theme engine (PHPTemplate), sub-themes will load their own
 *   template.php file in addition to the one used for its parent theme. This
 *   increases the risk for these errors. A good practice is to use the engine
 *   name for the base theme and the theme name for the sub-themes to minimize
 *   this possibility.
 *
 * - ENGINE_process_HOOK(&$variables)
 *   The same applies from the previous function, but it is called for a
 *   specific hook.
 *
 * - THEME_process(&$variables)
 *   These functions are based upon the raw theme; they should primarily be
 *   used by themes that do not use an engine or by sub-themes. It serves the
 *   same purpose as ENGINE_process().
 *
 * - THEME_process_HOOK(&$variables)
 *   The same applies from the previous function, but it is called for a
 *   specific hook.
 *
 * If the implementation is a function, only the hook-specific preprocess
 * and process functions (the ones ending in _HOOK) are called from the
 * above list. This is because theme hooks with function implementations
 * need to be fast, and calling the non-hook-specific preprocess and process
 * functions for them would incur a noticeable performance penalty.
 *
 * For template-implemented theme hooks, there are two special variables that
 * these preprocess and process functions can set:
 *   'template_file' and 'template_files'. These will be merged together
 *   to form a list of 'suggested' alternate template files to use, in
 *   reverse order of priority. template_file will always be a higher
 *   priority than items in template_files. theme() will then look for these
 *   files, one at a time, and use the first one that exists. If none exists,
 *   theme() will use the original registered file for the theme hook.
 *
 * For function-implemented theme hooks, there are two special variables that
 * these preprocess and process functions can set:
 *   'theme_function' and 'theme_functions'. These will be merged together
 *   to form a list of 'suggested' alternate functions to use, in
 *   reverse order of priority. theme_function will always be a higher
 *   priority than items in theme_functions. theme() will then call the
 *   highest priority function that exists. If none exists, theme() will call
 *   the original registered function for the theme hook.
 *
 * @param $hook
 *   The name of the theme function to call. May be an array, in which
 *   case the first hook that actually has an implementation registered
 *   will be used. This can be used to choose 'fallback' theme implementations,
 *   so that if the specific theme hook isn't implemented anywhere, a more
 *   generic one will be used. This can allow themes to create specific theme
 *   implementations for named objects.
 *
 * @param $variables
 *   An associative array of variables to merge with defaults from the theme
 *   registry, pass to preprocess and process functions for modification, and
 *   finally, pass to the function or template implementing the theme hook.
 *   Alternatively, this can be a renderable array, in which case, its properties
 *   are mapped to variables expected by the theme hook implementations.
 *
 * @return
 *   An HTML string that generates the themed output.
 */
function theme($hook, $variables = array()) {
  static $hooks = NULL;
  if (!isset($hooks)) {
    drupal_theme_initialize();
    $hooks = theme_get_registry();
  }

  // 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 (!isset($hooks[$hook])) {
    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 function/template should be used.
  if (isset($info['preprocess functions']) || isset($info['process functions'])) {
    $variables['theme_functions'] = array();
    $variables['template_files'] = 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);
          }
        }
      }
    }
    // Function suggestion takes priority over template suggestion.
    // theme_function takes priority over theme_functions.
    // theme_functions are in FILO order (least appropriate to most appropriate).
    // Here, just look for function suggestions. Deal with template
    // suggestions only after determining that the theme call is a template.
    $suggestions = array();
    if (!empty($variables['theme_functions'])) {
      $suggestions = $variables['theme_functions'];
    }
    if (!empty($variables['theme_function'])) {
      $suggestions[] = $variables['theme_function'];
    }
    foreach (array_reverse($suggestions) as $suggestion) {
      if (function_exists($suggestion)) {
        $info['function'] = $suggestion;
        break;
      }
    }
  }

  // Generate the output using either a function or a template.
  if (isset($info['function'])) {
    if (function_exists($info['function'])) {
      $output = $info['function']($variables);
    }
  }
  else {
    // Default render function and extension.
    $render_function = 'theme_render_template';
    $extension = '.tpl.php';

    // Run through the theme engine variables, if necessary
    global $theme_engine;
    if (isset($theme_engine)) {
      // If theme or theme engine is implementing this, it may have
      // a different extension and a different renderer.
      if ($info['type'] != 'module') {
        if (function_exists($theme_engine . '_render_template')) {
          $render_function = $theme_engine . '_render_template';
        }
        $extension_function = $theme_engine . '_extension';
        if (function_exists($extension_function)) {
          $extension = $extension_function();
        }
      }
    }

    // Find which template file exists and can be used. Priority order is:
    // 1. $variables['template_file'].
    // 2. $variables['template_files'] in FILO order (later in array is higher
    //    priority).
    // 3. $info['template'].
    $suggestions = array();
    if (isset($variables['template_files'])) {
      $suggestions = $variables['template_files'];
    }
    if (isset($variables['template_file'])) {
      $suggestions[] = $variables['template_file'];
    }
    if ($suggestions) {
      $template_file = drupal_discover_template($info['theme paths'], $suggestions, $extension);
    }
    if (empty($template_file)) {
      $template_file = $info['template'] . $extension;
      if (isset($info['path'])) {
        $template_file = $info['path'] . '/' . $template_file;
      }
    }

    // Render the output using the found template file.
    $output = $render_function($template_file, $variables);
  }

  // restore path_to_theme()
  $theme_path = $temp;
  return $output;
}

/**
 * Determine and return which template file will generate the output.
 *
 * This helper allows the theme system to pick the template at runtime instead
 * of build time.
 *
 * @see template_page_suggestions()
 * @see template_preprocess_block()
 *
 * @param $paths
 *   The paths where templates can be found. See _theme_process_registry()
 *   'theme paths' for more information.
 * @param $suggestions
 *   The possible template names. These are derived from
 *   $variables['template_files'] and $variables['template_file], defined by
 *   preprocess functions. Each file is checked on every path in the order of
 *   precedence defined by theme().
 * @return
 *   The filepath to the template that will generate the output. If none is
 *   found, then theme() will use the 'template' as set by
 *   _theme_process_registry().
 *
 * @see _theme_process_registry()
 * @see theme()
 */
function drupal_discover_template($paths, $suggestions, $extension = '.tpl.php') {
  global $theme_engine;

  // Remove slashes or null to prevent files from being included from
  // an unexpected location (especially on Windows servers).
  $extension = str_replace(array("/", "\\", "\0"), '', $extension);

  // Loop through all paths and suggestions in FIFO order.
  $suggestions = array_reverse($suggestions);
  $paths = array_reverse($paths);
  foreach ($suggestions as $suggestion) {
    if (!empty($suggestion)) {
      $suggestion = str_replace(array("/", "\\", "\0"), '', $suggestion);
      foreach ($paths as $path) {
        if (file_exists($file = $path . '/' . $suggestion . $extension)) {
          return $file;
        }
      }
    }
  }
}

/**
 * Return the path to the current themed element.
 *
 * It can point to the active theme or the module handling a themed implementation.
 * For example, when invoked within the scope of a theming call it will depend
 * on where the theming function is handled. If implemented from a module, it
 * will point to the module. If implemented from the active theme, it will point
 * to the active theme. When called outside the scope of a theming call, it will
 * always point to the active theme.
 */
function path_to_theme() {
  global $theme_path;

  if (!isset($theme_path)) {
    drupal_theme_initialize();
  }

  return $theme_path;
}

/**
 * Allow themes and/or theme engines to easily discover overridden theme functions.
 *
 * @param $cache
 *   The existing cache of theme hooks to test against.
 * @param $prefixes
 *   An array of prefixes to test, in reverse order of importance.
 *
 * @return $templates
 *   The functions found, suitable for returning from hook_theme;
 */
function drupal_find_theme_functions($cache, $prefixes) {
  $templates = array();
  $functions = get_defined_functions();

  foreach ($cache as $hook => $info) {
    foreach ($prefixes as $prefix) {
      if (!empty($info['pattern'])) {
        $matches = preg_grep('/^' . $prefix . '_' . $info['pattern'] . '/', $functions['user']);
        if ($matches) {
          foreach ($matches as $match) {
            $new_hook = str_replace($prefix . '_', '', $match);
            $arg_name = isset($info['variables']) ? 'variables' : 'render element';
            $templates[$new_hook] = array(
              'function' => $match,
              $arg_name => $info[$arg_name],
            );
          }
        }
      }
      if (function_exists($prefix . '_' . $hook)) {
        $templates[$hook] = array(
          'function' => $prefix . '_' . $hook,
        );
        // Ensure that the pattern is maintained from base themes to its sub-themes.
        // Each sub-theme will have their functions scanned so the pattern must be
        // held for subsequent runs.
        if (isset($info['pattern'])) {
          $templates[$hook]['pattern'] = $info['pattern'];
        }
      }
    }
  }

  return $templates;
}

/**
 * Allow themes and/or theme engines to easily discover overridden templates.
 *
 * @param $cache
 *   The existing cache of theme hooks to test against.
 * @param $extension
 *   The extension that these templates will have.
 * @param $path
 *   The path to search.
 */
function drupal_find_theme_templates($cache, $extension, $path) {
  $templates = array();

  // Collect paths to all sub-themes grouped by base themes. These will be
  // used for filtering. This allows base themes to have sub-themes in its
  // folder hierarchy without affecting the base themes template discovery.
  $theme_paths = array();
  foreach (list_themes() as $theme_info) {
    if (!empty($theme_info->base_theme)) {
      $theme_paths[$theme_info->base_theme][$theme_info->name] = dirname($theme_info->filename);
    }
  }
  foreach ($theme_paths as $basetheme => $subthemes) {
    foreach ($subthemes as $subtheme => $subtheme_path) {
      if (isset($theme_paths[$subtheme])) {
        $theme_paths[$basetheme] = array_merge($theme_paths[$basetheme], $theme_paths[$subtheme]);
      }
    }
  }
  global $theme;
  $subtheme_paths = isset($theme_paths[$theme]) ? $theme_paths[$theme] : array();

  // Escape the periods in the extension.
  $regex = '/' . str_replace('.', '\.', $extension) . '$/';
  // Because drupal_system_listing works the way it does, we check for real
  // templates separately from checking for patterns.
  $files = drupal_system_listing($regex, $path, 'name', 0);
  foreach ($files as $template => $file) {
    // Ignore sub-theme templates for the current theme.
    if (strpos($file->uri, str_replace($subtheme_paths, '', $file->uri)) !== 0) {
      continue;
    }
    // Chop off the remaining extensions if there are any. $template already
    // has the rightmost extension removed, but there might still be more,
    // such as with .tpl.php, which still has .tpl in $template at this point.
    if (($pos = strpos($template, '.')) !== FALSE) {
      $template = substr($template, 0, $pos);
    }
    // Transform - in filenames to _ to match function naming scheme
    // for the purposes of searching.
    $hook = strtr($template, '-', '_');
    if (isset($cache[$hook])) {
      $templates[$hook] = array(
        'template' => $template,
        'path' => dirname($file->uri),
      );
    }
    // Ensure that the pattern is maintained from base themes to its sub-themes.
    // Each sub-theme will have their templates scanned so the pattern must be
    // held for subsequent runs.
    if (isset($cache[$hook]['pattern'])) {
      $templates[$hook]['pattern'] = $cache[$hook]['pattern'];
    }
  }

  $patterns = array_keys($files);

  foreach ($cache as $hook => $info) {
    if (!empty($info['pattern'])) {
      // Transform _ in pattern to - to match file naming scheme
      // for the purposes of searching.
      $pattern = strtr($info['pattern'], '_', '-');

      $matches = preg_grep('/^' . $pattern . '/', $patterns);
      if ($matches) {
        foreach ($matches as $match) {
          $file = substr($match, 0, strpos($match, '.'));
          // Put the underscores back in for the hook name and register this pattern.
          $arg_name = isset($info['variables']) ? 'variables' : 'render element';
          $templates[strtr($file, '-', '_')] = array(
            'template' => $file,
            'path' => dirname($files[$match]->uri),
            $arg_name => $info[$arg_name],
          );
        }
      }
    }
  }
  return $templates;
}

/**
 * Retrieve a setting for the current theme or for a given theme.
 *