1 theme.inc theme($hook, $variables = array())

Generates themed output.

All requests for themed output must go through this function. It examines the request and routes it to the appropriate theme function or template, by checking the theme registry.

You can call this function directly, or you can make a call to backdrop_render() by passing a render array with a #theme key, which in turn calls theme(). Either is acceptable. In some situations a render array will be preferable when the ability for other modules to alter the render array is provided.

Theme Hooks

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'.

Overriding Theme Hooks

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).

Preprocessing for Template Files

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.
  • 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.

Preprocessing for Theme Functions

If the implementation is a function, only the theme-hook-specific preprocess 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 functions for them would incur a noticeable performance penalty.

Suggesting Alternate Hooks

There are two special variables that these preprocess 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 function.


$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.

$variables: An associative array of variables to merge with defaults from the theme registry, pass to preprocess 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 value

string: An HTML string representing the themed output.



See also


Default theme implementations




core/includes/theme.inc, line 1016
The theme system, which controls the output of Backdrop.


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])) {
    $hook = $candidate;
  $theme_hook_original = $hook;

  // 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[$hook])) {
    if (!isset($hooks[$hook])) {
      // Only log a message when not trying theme suggestions ($hook being an
      // array).
      if (!isset($candidate)) {
        watchdog('theme', 'Theme hook "@hook" not found.', array('@hook' => $hook), WATCHDOG_WARNING);
      return '';

  $info = $hooks[$hook] + array(
    'includes' => array(),
    'preprocess functions' => array(),

  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 preprocessor is held
  // elsewhere.
  foreach ($info['includes'] as $include_file) {
    include_once BACKDROP_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());

  $variables['theme_hook_original'] = $theme_hook_original;

  // Invoke the variable preprocessors, if any. The preprocessors 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 preprocessors of
  // the base hook, but retain the suggestion as a high priority suggestion to
  // be used unless overridden by a variable preprocessor function.
  if (isset($info['base hook'])) {
    $base_hook = $info['base hook'];
    $base_hook_info = isset($hooks[$base_hook]) ? $hooks[$base_hook] : array();
    $base_hook_info += array(
      'includes' => array(),
      'preprocess functions' => array(),
    // Include files required by the base hook, since its variable preprocessors
    // might reside there.
    foreach ($base_hook_info['includes'] as $include_file) {
      include_once BACKDROP_ROOT . '/' . $include_file;
    $variables['theme_hook_suggestion'] = $hook;
    $hook = $base_hook;

    // Combine preprocess functions to call the base hook and child functions.
    $info['preprocess functions'] = array_unique(array_merge($base_hook_info['preprocess functions'], $info['preprocess functions']));
  if (isset($info['preprocess functions'])) {
    $variables['theme_hook_suggestions'] = array();
    foreach ($info['preprocess functions'] as $processor_function) {
      if (function_exists($processor_function)) {
        // We don't want a poorly behaved preprocess function changing $hook.
        $hook_clone = $hook;
        $processor_function($variables, $hook_clone);
    // If the preprocess 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 step to route to a
    // more specific theme hook. For example, a function may call
    // theme('node', ...), but a preprocess function can add 'node__post' as
    // a suggestion, enabling a theme to have an alternate template file for
    // post nodes. Suggestions are checked in the following order:
    // - The 'theme_hook_suggestion' variable is checked first. It overrides
    //   all others.
    // - The 'theme_hook_suggestions' variable is checked in FILO order, so the
    //   last suggestion added to the array takes precedence over suggestions
    //   added earlier.
    $suggestions = array();
    if (!empty($variables['theme_hook_suggestions'])) {
      $suggestions = $variables['theme_hook_suggestions'];
    if (!empty($variables['theme_hook_suggestion'])) {
      $suggestions[] = $variables['theme_hook_suggestion'];
    foreach (array_reverse($suggestions) as $suggestion) {
      if (isset($hooks[$suggestion])) {
        $info = $hooks[$suggestion];

  // Generate the output using either a function or a template.
  $output = '';
  if (isset($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';

    // The theme engine may use a different extension and a different renderer.
    global $theme_engine;
    if (isset($theme_engine)) {
      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();

    // In some cases, a template implementation may not have had
    // template_preprocess() run (for example, if the default implementation is
    // a function, but a template overrides that default implementation). In
    // these cases, a template should still be able to expect to have access to
    // the variables provided by template_preprocess(), so we add them here if
    // they don't already exist. We don't want to run template_preprocess()
    // twice (it would be inefficient and mess up zebra striping), so we use the
    // 'directory' variable to determine if it has already run, which while not
    // completely intuitive, is reasonably safe, and allows us to save on the
    // overhead of adding some new variable to track that.
    if (!isset($variables['directory'])) {
      $default_template_variables = array();
      template_preprocess($default_template_variables, $hook);
      $variables += $default_template_variables;

    // Render the output using the template file.
    $template_file = $info['template'] . $extension;
    if (isset($info['path'])) {
      $template_file = $info['path'] . '/' . $template_file;
    try {
      $theme_debug = config_get('system.core', 'theme_debug');
    catch (ConfigException $e) {
      $theme_debug = FALSE;
    if ($theme_debug) {
      $output = _theme_render_template_debug($render_function, $template_file, $variables, $extension);
    else {
      $output = $render_function($template_file, $variables);

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