Posts, Post types and Taxonomy changes in WordPress 5.9

In WordPress 5.9, new hooksHooks In WordPress theme and development, hooks are functions that can be applied to an action or a Filter in WordPress. Actions are functions performed when a certain event occurs in WordPress. Filters allow you to modify certain functions. Arguments used to hook both filters and actions look the same. and functions are added to help developers to work with Posts, Post types and Taxonomies.

is_post_type_viewable filterFilter Filters are one of the two types of Hooks They provide a way for functions to modify data of other functions. They are the counterpart to Actions. Unlike Actions, filters are meant to work in an isolated manner, and should never have side effects such as affecting global variables and output.

WP 5.9 introduces the new is_post_type_viewable filter to allow developers to hook into is_post_type_viewable() to override the check performed by this function.

This filter exposes the $post_type object to allow to return either true or false depending on their needs. The expected filtered value is a boolean. As filtered values can change, including the data type, this commit includes a is_bool() check, thus ensuring backwards-compatibility and guard against potential type errors in PHPPHP The web scripting language in which WordPress is primarily architected. WordPress requires PHP 5.6.20 or higher 8.1+. Non-boolean values (even falsey and truthy values) will result in the function returning false.


 * Override is_post_type_viewable() value for the "Books" custom post type. 
function wporg_books_is_not_a_viewable_post_type( $is_viewable, $post_type ) {
	if ( __( 'Books', 'my-plugin' ) === $post_type->label ) {
		return false;
	return $is_viewable;
add_filter( 'is_post_type_viewable', 'wporg_books_is_not_a_viewable_post_type', 10, 2 );

Related ticketticket Created for both bug reports and feature development on the bug tracker. on TracTrac An open source project by Edgewall Software that serves as a bug tracker and project management tool for WordPress.: #49628.

is_post_status_viewable filter

Similarly to is_post_type_viewable, the is_post_status_viewable filter allow developers to hook into the related PHP function. This filter exposes the $post_status object to allow to return either true or false depending on their needs.


 * Override is_post_status_viewable() value for the "Unread" custom post status. 
function wporg_unread_is_not_a_viewable_post_status( $is_viewable, $post_status ) {
	if ( __( 'Unread', 'my-plugin' ) === $post_status->label ) {
		return false;
	return $is_viewable;
add_filter( 'is_post_status_viewable', 'wporg_unread_is_not_a_viewable_post_status', 10, 2 );

Related ticket on Trac: #54375.

post_thumbnail_url filter

WP 5.9 Introduces the new filter post_thumbnail_url which allows overriding the default url returned from wp_get_attachement_image_url() function. It passes the following parameters:

  • $thumbnail_url: The Post thumbnail URLURL A specific web address of a website or web page on the Internet, such as a website’s URL (or false if the post does not exist)
  • $post: The Post ID or WP_Post object. Default is global $post
  • $size: The Registered image size to retrieve the source for or a flat array of height and width dimensions. Default value: post-thumbnail


 * Override the post thumbnail URL for a specific template.
function wporg_change_post_thumbnail_url_for_about_template( $thumbnail_url, $post, $size ) {
	if ( 'templates/about.php' !== get_page_template_slug( $post ) ) {
		return wp_get_attachment_image_url( get_template_directory . '/images/my-specific-image.png' );
	return $thumbnail_url;
add_filter( 'post_thumbnail_url', 'wporg_change_post_thumbnail_url_for_about_template', 10, 3 );

Related ticket on Trac: #40547.

post_thumbnail_id filter

Similarly, WP 5.9 introduces the new post_thumbnail_id filter which allows overriding the default id returned from get_post_thumbnail_id(). It passes the following parameters:

  • $thumbnail_id: The Post thumbnail ID (or false if the post does not exist)
  • $post: The Post ID or WP_Post object. Default is global $post

Related ticket on Trac: #23983.

New labels available in register_taxonomy()

In WP 5.9, some static strings were replaced with additional label options to allow developers further flexibility for customizing the Edit {taxonomyTaxonomy A taxonomy is a way to group things together. In WordPress, some common taxonomies are category, link, tag, or post format.} screen.

The following labels were added:

  • name_field_description: Description for the Name field on Edit Tags screen. Default: “The name is how it appears on your site.”
  • parent_field_description: Description for the Parent field on Edit Tags screen. Default: “Assign a parent term to create a hierarchy. The term Jazz, for example, would be the parent of Bebop and Big Band.”
  • slug_field_description: Description for the Slug field on Edit Tags screen. Default: “The « slug » is the URL-friendly version of the name. It is usually all lowercase and contains only letters, numbers, and hyphens.”
  • desc_field_description: Description for the Description field on Edit Tags screen. Default: “The description is not prominent by default; however, some themes may show it.”

Related ticket on Trac: #43060.

New function to get the URL for existing revisionsRevisions The WordPress revisions system stores a record of each saved draft or published update. The revision system allows you to see what changes were made in each revision by dragging a slider (or using the Next/Previous buttons). The display indicates what has changed in each revision. of a post: wp_get_post_revisions_url()

Since WP 5.9, the wp_get_post_revisions_url() function can be used to get a link to a given post’s revisions.


  • $post_id (optional): Post ID or WP_Post object. Default is global $post.

This function returns the URL for editing revisions on the given post (or null otherwise).

Related ticket on Trac: #39062.

New built-in post types in WP 5.9

Please note that WordPress 5.9 introduces four new built-in post types related to the new full site editing experience and are used when a BlockBlock Block is the abstract term used to describe units of markup that, composed together, form the content or layout of a webpage using the WordPress editor. The idea combines concepts of what in the past may have achieved with shortcodes, custom HTML, and embed discovery into a single consistent API and user experience. Theme is activated.

  • wp_template: The templates to include in your theme.
  • wp_template_part: The template parts to include in your templates.
  • wp_global_styles: The styles created and saved by the site adminadmin (and super admin) for the current theme.
  • wp_navigation: The navigation menus that can be inserted into the site.

Additional dev notesdev note Each important change in WordPress Core is documented in a developers note, (usually called dev note). Good dev notes generally include a description of the change, the decision that led to this change, and a description of how developers are supposed to work with that change. Dev notes are published on Make/Core blog during the beta phase of WordPress release cycle. Publishing dev notes is particularly important when plugin/theme authors and WordPress developers need to be aware of those changes.In general, all dev notes are compiled into a Field Guide at the beginning of the release candidate phase. will be published to introduce the new full site editing experience. Note the above post types are reserved terms for WordPress internal usage.

Thanks @mkaz for proofreading.

#5-9, #dev-notes