This document discusses WordPress filters and provides examples of how they can be used. It begins by defining a filter as a function that takes a WordPress value, optionally modifies it, and returns the resulting value. It then explains the apply_filters() function and lists over 1,300 filter hooks in core. Examples are given for using filters to modify content in the editor, customize admin menu ordering, filter meta boxes and post types when deleting users. Front-end filters discussed include modifying post content, body classes, and excerpt length. The document concludes with filters useful for development like modifying plugins/themes listings and enabling post by email.

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THERE’S A FILTER FORTHAT
Drew Jaynes | WCABQ ’13

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WHO I AM
Drew Jaynes
Web Engineer, 10up
Core Contributor
WordPress Docs Team

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WHAT IS A FILTER?
A filter is a function that:
1.Takes a specific WordPress value
2. (maybe) modifies it
3. Spits the resulting value back out

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OK ... ?

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apply_filters()

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apply_filters()
– 1,300+ filter hooks in core (3.6+)
– Filter values of all types, modify default behavior
– Multiple filters run per hook, based on priority

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ADDING OR REMOVING FILTERS
add_filter( 'hook_name', 'callback', #priority, #args );
– When removing a filter, the hook, callback and priority must
match exactly. Fails silently.
remove_filter( 'hook_name', 'callback', #priority );
– Define priority any time. Define the args count + 1 if
additional args are passed from the filter hook

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ALLTOGETHER NOW
// $value is filterable
apply_filters( ‘hook_name’, $value, (optional) $args );
function callback( $value, $arg ) {
// do stuff
return $value;
}
add_filter( 'hook_name', 'callback', 10, 2 );

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CONTENT

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‘enter_title_here’
Args:(string $enter_title_here, WP_Post $post)
– Match title prompt text to your custom post type
– Adds polish to the experience
Filter ‘Enter title here’ placeholder text

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‘post_updated_messages’
Args:(array $messages)
– Not handled by WordPress out of the box
– Use with custom post types
– Customize ‘Post updated’,‘Post deleted’, etc.
Filter post updated|published|trashed|etc. messages.

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‘admin_post_thumbnail_html’
Args:(string $content, int $post->ID)
‘media_view_strings’
Args:(array $strings, WP_Post $post)
Filter HTML output of Featured Image meta box
Filter media strings localized for JavaScript

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WORKFLOW

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‘the_editor_content’
Args:(string $content)
– Pre-fab post template
– Check if the content is empty
Filter the content in the editor (new and existing posts)

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‘custom_menu_order’
Args:(bool false)
Enable/disable custom admin menu ordering
‘menu_order’
Args:(array $menu_order)
Filter the top-level menu order in the admin menu

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‘custom_menu_order’
// enable custom menu order
add_filter( ‘custom_menu_order’, __return_true’ );

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‘menu_order’
function change_menu_order( $menu_order );
return array(
'index.php',
'separator1',
'edit.php', 'edit.php?post_type=page',
'users.php', 'upload.php',
'edit-comments.php',
'separator2',
'themes.php', 'plugins.php',
'tools.php', 'options-general.php',
'separator-last'
);
}
add_filter( ‘menu_order’, ‘change_menu_order’ );

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‘default_hidden_meta_boxes’
Args:(array $hidden, WP_Screen $screen)
– Standardize UX for all users
– Doesn’t prevent future user changes
Contextually filter meta boxes hidden by default

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‘post_types_to_delete_with_user’
Args:(array $post_types_to_delete, int $id)
– Overrides ‘delete_with_user’ post type setting
Filter which post types to delete with a user

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FRONT-END

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‘the_content’
(string $content)
– Frequently used and abused in themes and plugins
– Litmus test: is it content?
Filter the post content

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‘post|body_class’
Args:(array $classes)
– Reliant on themes using post|body_class() in templates
– Style things in specific contexts, e.g. post_type, post format, etc.
Filter body classes or post classes

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‘excerpt_length’
Args:(int $number)
– Commonly filtered by themes, default is 55 characters
Filter the length (in characters) of the excerpt

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‘show_admin_bar’
(bool true)
// Disable Toolbar
add_filter( ‘show_admin_bar’, ‘__return_false’ );
Show/hide theToolbar on the front-end

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‘disable_captions’
(bool false)
// Disable captions
add_filter( ‘disable_captions’, ‘__return_true’ );
Disable/enable image captions

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DEVELOPMENT

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‘plugins|themes_api_*’
– Different result object, arguments, or complete overhaul
Filter plugin or theme installer pages
‘plugins|themes_api_args’
‘plugins|themes_api’
‘plugins|themes_api_result’

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‘enable_post_by_email_configuration’
Args:(bool false)
// disable post by email
add_filter(
‘enable_post_by_email_configuration’,
‘__return_false’
);
Enable/disable Post by Email

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MOAR DEV FILTERS
‘request’
Args:(array $query_vars)
Directly filter the current request’s query vars
‘shortcode_atts_$shortcode’
Args:(array $out, array $pairs, array $atts)
Filter a shortcode’s default attributes (3.6+)
– Output atts, default atts, user-defined atts

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MOAR DEV FILTERS
‘wp_xmlrpc_server_class’
Args:(string $class)
Filter the class used for handling XML-RPC requests
‘xmlrpc_methods’
Args:(array $methods)
Register custom XML-RPC methods

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QUESTIONS?