Hooks in Drupal

Allow modules to interact with the Drupal core.
Drupal's module system is based on the concept of "hooks". A hook is a PHP function that is named foo_bar(), where "foo" is the name of the module (whose filename is thus foo.module) and "bar" is the name of the hook. Each hook has a defined set of parameters and a specified result type.
To extend Drupal, a module need simply implement a hook. When Drupal wishes to allow intervention from modules, it determines which modules implement a hook and calls that hook in all enabled modules that implement it.
The available hooks to implement are explained here in the Hooks section of the developer documentation. The string "hook" is used as a placeholder for the module name in the hook definitions. For example, if the module file is called example.module, then hook_help() as implemented by that module would be defined as example_help().

Functions & methods

Name	Description

Name	Description
custom_url_rewrite_inbound	custom_url_rewrite_inbound is not a hook, it's a function you can add to settings.php to alter incoming requests so they map to a Drupal path. This function is called before modules are loaded and the menu system is initialized and it changes…
custom_url_rewrite_outbound	custom_url_rewrite_outbound is not a hook, it's a function you can add to settings.php to alter all links generated by Drupal. This function is called from url(). This function is called very frequently (100+ times per page) so performance…
hook_access	Define access restrictions.
hook_actions_delete	Execute code after an action is deleted.
hook_action_info	Declare information about one or more Drupal actions.
hook_action_info_alter	Alter the actions declared by another module.
hook_block	Declare a block or set of blocks.
hook_boot	Perform setup tasks. See also, hook_init.
hook_comment	Respond to comment actions.
hook_cron	Perform periodic actions.
hook_db_rewrite_sql	Rewrite database queries, usually for access control.
hook_delete	Respond to node deletion.
hook_disable	Perform necessary actions before module is disabled.
hook_elements	Allows modules to declare their own Forms API element types and specify their default values.
hook_enable	Perform necessary actions after module is enabled.
hook_exit	Perform cleanup tasks.
hook_file_download	Control access to private file downloads and specify HTTP headers.
hook_filter	Define content filters.
hook_filter_tips	Provide tips for using filters.
hook_flush_caches	Add a list of cache tables to be cleared.
hook_footer	Insert closing HTML.
hook_form	Display a node editing form.
hook_forms	Map form_ids to builder functions.
hook_form_alter	Perform alterations before a form is rendered.
hook_form_FORM_ID_alter	Provide a form-specific alteration instead of the global hook_form_alter().
hook_help	Provide online user help.
hook_hook_info	Expose a list of triggers (events) that users can assign actions to.
hook_init	Perform setup tasks. See also, hook_boot.
hook_insert	Respond to node insertion.
hook_install	Install the current version of the database schema, and any other setup tasks.
hook_link	Define internal Drupal links.
hook_link_alter	Perform alterations before links on a node or comment are rendered.
hook_load	Load node-type-specific information.
hook_locale	Allows modules to define their own text groups that can be translated.
hook_mail	Prepare a message based on parameters; called from drupal_mail().
hook_mail_alter	Alter any aspect of email sent by Drupal. You can use this hook to add a common site footer to all outgoing email, add extra header fields, and/or modify the email in any way. HTML-izing the outgoing email is one possibility. See also drupal_mail().
hook_menu	Define menu items and page callbacks.
hook_menu_alter	Alter the data being saved to the {menu_router} table after hook_menu is invoked.
hook_menu_link_alter	Alter the data being saved to the {menu_links} table by menu_link_save().
hook_nodeapi	Act on nodes defined by other modules.
hook_node_access_records	Set permissions for a node to be written to the database.
hook_node_grants	Inform the node access system what permissions the user has.
hook_node_info	Define module-provided node types.
hook_node_operations	Add mass node operations.
hook_node_type	Act on node type changes.
hook_openid	Allow modules to modify the OpenID request parameters.
hook_perm	Define user permissions.
hook_ping	Ping another server.
hook_prepare	This is a hook used by node modules. It is called after load but before the node is shown on the add/edit form.
hook_profile_alter	Alter profile items before they are rendered.
hook_requirements	Check installation requirements and do status reporting.
hook_schema	Define the current version of the database schema.
hook_schema_alter	Perform alterations to existing database schemas.
hook_search	Define a custom search routine.
hook_search_preprocess	Preprocess text for the search index.
hook_system_info_alter	Alter the information parsed from module and theme .info files
hook_taxonomy	Act on taxonomy changes.
hook_term_path	Allows modules to provide an alternative path for the terms it manages.
hook_theme	Register a module (or theme's) theme implementations.
hook_theme_registry_alter	Alter the theme registry information returned from hook_theme().
hook_translated_menu_link_alter	Alter a menu link after it's translated, but before it's rendered.
hook_translation_link_alter	Perform alterations on translation links.
hook_uninstall	Remove any information that the module sets.
hook_update	Respond to node updating.
hook_update_index	Update Drupal's full-text index for this module.
hook_update_last_removed	Return a number which is no longer available as hook_update_N().
hook_update_N	Perform a single update.
hook_update_projects_alter	Alter the list of projects before fetching data and comparing versions.
hook_update_status_alter	Alter the information about available updates for projects.
hook_user	Act on user account actions.
hook_user_operations	Add mass user operations.
hook_validate	Verify a node editing form.
hook_view	Display a node.
hook_watchdog	Log an event message
hook_xmlrpc	Register XML-RPC callbacks.
module_hook	Determine whether a module implements a hook.
module_implements	Determine which modules are implementing a hook.
module_invoke	Invoke a hook in a particular module.
module_invoke_all	Invoke a hook in all enabled modules that implement it.
page_cache_fastpath	Outputs a cached page.