How WordPress understands what do with the (all) key in the $wp_filter array? [duplicate]

You have all explained in this function for instance

File: wp-includes/plugin.php
402: /**
403:  * Execute functions hooked on a specific action hook.
404:  *
405:  * This function invokes all functions attached to action hook `$tag`. It is
406:  * possible to create new action hooks by simply calling this function,
407:  * specifying the name of the new hook using the `$tag` parameter.
408:  *
409:  * You can pass extra arguments to the hooks, much like you can with apply_filters().
410:  *
411:  * @since 1.2.0
412:  *
413:  * @global array $wp_filter         Stores all of the filters
414:  * @global array $wp_actions        Increments the amount of times action was triggered.
415:  * @global array $wp_current_filter Stores the list of current filters with the current one last
416:  *
417:  * @param string $tag     The name of the action to be executed.
418:  * @param mixed  $arg,... Optional. Additional arguments which are passed on to the
419:  *                        functions hooked to the action. Default empty.
420:  */
421: function do_action($tag, $arg = '') {
422:    global $wp_filter, $wp_actions, $wp_current_filter;
423: 
424:    if ( ! isset($wp_actions[$tag]) )
425:        $wp_actions[$tag] = 1;
426:    else
427:        ++$wp_actions[$tag];
428: 
429:    // Do 'all' actions first
430:    if ( isset($wp_filter['all']) ) {
431:        $wp_current_filter[] = $tag;
432:        $all_args = func_get_args();
433:        _wp_call_all_hook($all_args);
434:    }
435: 
436:    if ( !isset($wp_filter[$tag]) ) {
437:        if ( isset($wp_filter['all']) )
438:            array_pop($wp_current_filter);
439:        return;
440:    }
441: 
442:    if ( !isset($wp_filter['all']) )
443:        $wp_current_filter[] = $tag;
444: 
445:    $args = array();
446:    if ( is_array($arg) && 1 == count($arg) && isset($arg[0]) && is_object($arg[0]) ) // array(&$this)
447:        $args[] =& $arg[0];
448:    else
449:        $args[] = $arg;
450:    for ( $a = 2, $num = func_num_args(); $a < $num; $a++ )
451:        $args[] = func_get_arg($a);
452: 
453:    $wp_filter[ $tag ]->do_action( $args );
454: 
455:    array_pop($wp_current_filter);
456: }

The part:

429:    // Do 'all' actions first

Speaks all has the special treatment inside the hook generator functions.

The similar logic you will find in other hook generator functions:

apply_filters
apply_filters_ref_array
apply_filters_deprecated
do_action
do_action_ref_array
do_action_deprecated

So all has the special treatment.

You may use all for introspection as in here where I list all actions hooks in sequence order for a single WordPress request.

From the comments:

I understand, but I want to know how WordPress understands that when passing this key all it will affect all actions or filters. It should be defined somewhere

Took me some time to understand what you meant. Yes. There is a global called $wp_filter in there all hooks are saved. In case of the example
add_action( 'all', '_20161224_printer' ); here, this will be inside that global.

[all] => WP_Hook Object
        (
            [callbacks] => Array
                (
                    [10] => Array
                        (
                            [_20161224_printer] => Array
                                (
                                    [function] => _20161224_printer
                                    [accepted_args] => 1
                                )

                        )

                )

            [iterations:WP_Hook:private] => Array
                (
                )

            [current_priority:WP_Hook:private] => Array
                (
                )

            [nesting_level:WP_Hook:private] => 0
            [doing_action:WP_Hook:private] => 
        )

Related Posts:

  1. Is there a way to send HTML formatted emails with WordPress’ wp_mail() function?
  2. Action hook for custom tax edit
  3. Hook after image is uploaded and image sizes generated
  4. Auto-retrieve YouTube Image for Thumbnail?
  5. How to call a REST endpoint when a post is published?
  6. __NAMESPACE__ with register_activation_hook
  7. Hook for URL Request
  8. Hook/notify when any option or setting is added or updated
  9. Which hooks is this? add_action(‘wp’,
  10. admin_notices after register_uninstall / deactivate_hook
  11. add_action on inherit post status
  12. Why does wp_enqueue_script ignore my ‘wp_head’ hook?
  13. How to add hook to the_title() and get_the_title()
  14. action lifecycle
  15. Redirect users on specific post category or category page
  16. Returning ACF custom field from publish_post
  17. How to modify an add_action() inside a loop of core function
  18. Hooking after get_header()
  19. How to get post ID in post_updated action hook?
  20. How can I set status=’publish’ for all featured images?
  21. What is the best filter where to use register_block_type?
  22. How get list all users who edited post?
  23. Stuck in redirect loop after using wp_login action
  24. Hooks for Links Box [duplicate]
  25. Check if do_action(‘custom_action’) is hooked into?
  26. Hook after wp_enqueue_scripts
  27. wp_login Action hook with conditional tag
  28. Can I remove WooCommerce main content hook?
  29. How to alter the query using pre_get_posts hook and is_post_type_archive
  30. How to update WordPress core or themes and still have my child theme hooks work
  31. Proper indentation of code generated inside hooks
  32. Add parameter (time) to oembed
  33. Event Calendar using insert_post_data after save_post and insert_post after post_transition draft_to_schedule
  34. How can I count post views of REST API calls and update them in an ACF field?
  35. Hooking into the init action will fire it too frequently?
  36. Event-Driven Pattern vs MVC?
  37. Using hooks with extra parameters
  38. How to catch and modify custom field values when a page is updated
  39. Genesis: How to add content after aside and before the content-sidebar wrap
  40. new_to_publish fires multiple times
  41. Plugin init hook
  42. Best action hook for placing ical requests
  43. Is there a hook that fires when a row of wp_sitemeta table is updated?
  44. `rest_user_query` can’t access post author in post edit screen
  45. Post Meta Emtpy on Publish Using Transition
  46. How to check post type when using sanitize_title hook?
  47. How do I trigger a post update within a get_posts() foreach loop?
  48. Duplicate Cron Jobs Using wp_next_scheduled / wp_schedule_event
  49. Hook when editing user
  50. get_post_metadata causing some meta data to fail
  51. How to change the default mail when admin approuved an user?
  52. How to modify how the_content outputs a link to an internal page
  53. read more, even if excerpt not trimmed
  54. How do I prevent term from being created on create_term hook?
  55. How to distinguish on hook profile_update user registering, user resetting password or user updating profile?
  56. Hook function prints output twice
  57. Hook priority in admin with custom plugin
  58. Embeding style into the header via the function.php
  59. ‘save_post’ hook not working in WP 3.5
  60. Change the Default Plugin page filter to Active intead of All
  61. How to run a function after wp() in the wp-blog-header.php file?
  62. How to execute a hook asynchronously?
  63. Issues with if, else, and elseif statements
  64. Is there a before_save_post hook or something similar
  65. wp query add array by if condition
  66. “Seen by” Feature in posts [closed]
  67. How to hook in to the “output buffer” and add html content
  68. hook for loading page
  69. What hook to use to redirect based on $post
  70. How can I modify the header of RSS feed items?
  71. Print results of a WP_Query to wp-admin/upload.php within ‘delete_attachment’ hook?
  72. upgrader_process_complete filter not fired using ajax updates
  73. How to debug wordpress hooked functions?
  74. remove_action(‘save_post’) in function that was triggered by save post not working
  75. How to get template name or category before publish
  76. WordPress How to reorder Page/Post Settings
  77. Is there a way to Insert a post to post list?
  78. “Intercept” enqueing of 3rd party’s JS file
  79. WP hook to add custom content after the_title()
  80. Struggling with plugin dev basics: add_action
  81. RTrouble passing arguments to action
  82. delete_user hook failed
  83. Prefixing widget_posts_args Hook
  84. wp_get_current_user Not working
  85. How can I insert custom html code inside a div dynamically?
  86. Display custom debug info inside page and not before head
  87. calculate values from a field and insert them into custom fields when publishing or updating post
  88. Why the post ID is 0 in a hook within wp_insert_post_data?
  89. WP-Automatic to run publish hooks
  90. template_redirect and title
  91. Hide post completely and still reach it via cURL
  92. How do I remove an action hook by s2member
  93. Problem in register activation hook and Copying folder
  94. Why does hook priority affects admin menu permission error?
  95. add_action failed to display function by a plugin
  96. WP: Override/update the_content from database with the publicly echo the_content
  97. What is the equivalent hook for both admin_head() and wp_head() combined, if any?
  98. Get post ID before add_theme_support
  99. Search WordPress Hook for completed Elementor Update
  100. How to filter the URL of thumbnail size in the Media Grid and admin pages