/Drupal 8

function hook_theme

hook_theme($existing, $type, $theme, $path)

Register a module or theme's theme implementations.

The implementations declared by this hook specify how a particular render array is to be rendered as HTML.


array $existing: An array of existing implementations that may be used for override purposes. This is primarily useful for themes that may wish to examine existing implementations to extract data (such as arguments) so that it may properly register its own, higher priority implementations.

$type: Whether a theme, module, etc. is being processed. This is primarily useful so that themes tell if they are the actual theme being called or a parent theme. May be one of:

  • 'module': A module is being checked for theme implementations.
  • 'base_theme_engine': A theme engine is being checked for a theme that is a parent of the actual theme being used.
  • 'theme_engine': A theme engine is being checked for the actual theme being used.
  • 'base_theme': A base theme is being checked for theme implementations.
  • 'theme': The actual theme in use is being checked.

$theme: The actual name of theme, module, etc. that is being being processed.

$path: The directory path of the theme or module, so that it doesn't need to be looked up.

Return value

array An associative array of information about theme implementations. The keys on the outer array are known as "theme hooks". For theme suggestions, instead of the array key being the base theme hook, the key is a theme suggestion name with the format 'base_hook_name__sub_hook_name'. For render elements, the key is the machine name of the render element. The array values are themselves arrays containing information about the theme hook and its implementation. Each information array must contain either a 'variables' element (for using a #theme element) or a 'render element' element (for render elements), but not both. The following elements may be part of each information array:

  • variables: Only used for #theme in render array: an array of variables, where the array keys are the names of the variables, and the array values are the default values if they are not given in the render array. Template implementations receive each array key as a variable in the template file (so they must be legal PHP/Twig variable names). Function implementations are passed the variables in a single $variables function argument. If you are using these variables in a render array, prefix the variable names defined here with a #.
  • render element: Used for render element items only: the name of the renderable element or element tree to pass to the theme function. This name is used as the name of the variable that holds the renderable element or tree in preprocess and process functions.
  • file: The file the implementation resides in. This file will be included prior to the theme being rendered, to make sure that the function or preprocess function (as needed) is actually loaded.
  • path: Override the path of the file to be used. Ordinarily the module or theme path will be used, but if the file will not be in the default path, include it here. This path should be relative to the Drupal root directory.
  • template: If specified, the theme implementation is a template file, and this is the template name. Do not add 'html.twig' on the end of the template name. The extension will be added automatically by the default rendering engine (which is Twig.) If 'path' is specified, 'template' should also be specified. If neither 'template' nor 'function' are specified, a default template name will be assumed. For example, if a module registers the 'search_result' theme hook, 'search-result' will be assigned as its template name.
  • function: (deprecated in Drupal 8.0.x, will be removed in Drupal 9.0.x) If specified, this will be the function name to invoke for this implementation. If neither 'template' nor 'function' are specified, a default template name will be assumed. See above for more details.
  • base hook: Used for theme suggestions only: the base theme hook name. Instead of this suggestion's implementation being used directly, the base hook will be invoked with this implementation as its first suggestion. The base hook's files will be included and the base hook's preprocess functions will be called in place of any suggestion's preprocess functions. If an implementation of hook_theme_suggestions_HOOK() (where HOOK is the base hook) changes the suggestion order, a different suggestion may be used in place of this suggestion. If after hook_theme_suggestions_HOOK() this suggestion remains the first suggestion, then this suggestion's function or template will be used to generate the rendered output.
  • pattern: A regular expression pattern to be used to allow this theme implementation to have a dynamic name. The convention is to use __ to differentiate the dynamic portion of the theme. For example, to allow forums to be themed individually, the pattern might be: 'forum__'. Then, when the forum is rendered, following render array can be used:
    $render_array = array(
      '#theme' => array('forum__' . $tid, 'forum'),
      '#forum' => $forum,
  • preprocess functions: A list of functions used to preprocess this data. Ordinarily this won't be used; it's automatically filled in. By default, for a module this will be filled in as template_preprocess_HOOK. For a theme this will be filled in as twig_preprocess and twig_preprocess_HOOK as well as themename_preprocess and themename_preprocess_HOOK.
  • override preprocess functions: Set to TRUE when a theme does NOT want the standard preprocess functions to run. This can be used to give a theme FULL control over how variables are set. For example, if a theme wants total control over how certain variables in the page.html.twig are set, this can be set to true. Please keep in mind that when this is used by a theme, that theme becomes responsible for making sure necessary variables are set.
  • type: (automatically derived) Where the theme hook is defined: 'module', 'theme_engine', or 'theme'.
  • theme path: (automatically derived) The directory path of the theme or module, so that it doesn't need to be looked up.

See also

Theme system overview


Related topics

Define functions that alter the behavior of Drupal core.


core/lib/Drupal/Core/Render/theme.api.php, line 1181
Hooks and documentation related to the theme and render system.


function hook_theme($existing, $type, $theme, $path) {
  return array(
    'forum_display' => array(
      'variables' => array('forums' => NULL, 'topics' => NULL, 'parents' => NULL, 'tid' => NULL, 'sortby' => NULL, 'forum_per_page' => NULL),
    'forum_list' => array(
      'variables' => array('forums' => NULL, 'parents' => NULL, 'tid' => NULL),
    'forum_icon' => array(
      'variables' => array('new_posts' => NULL, 'num_posts' => 0, 'comment_mode' => 0, 'sticky' => 0),
    'status_report' => array(
      'render element' => 'requirements',
      'file' => 'system.admin.inc',

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