Edit 7/29/2020: a call out specific to Twenty Ten was added for the comment_type change.

WordPress 5.5 comes with a number of small developer-focused changes. Here’s a summary of what you can expect.

Upgrade/Install: updating a pluginPlugin A plugin is a piece of software containing a group of functions that can be added to a WordPress website. They can extend functionality or add new features to your WordPress websites. WordPress plugins are written in the PHP programming language and integrate seamlessly with WordPress. These can be free in the WordPress.org Plugin Directory https://wordpress.org/plugins/ or can be cost-based plugin from a third-party or theme by uploading a ZIP file

Starting in WordPress 5.5, it will be possible to update a plugin or theme by uploading a ZIP file. After uploading the ZIP file, a comparison table will be displayed to the user, detailing the differences between the uploaded ZIP and the version of the plugin currently installed.

The majority of this process is handled by the new do_overwrite() methods within the Theme_Installer_Skin and Plugin_Installer_Skin classes. However, there are a few 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. that can be used to adjust the output of this feature:

• The install_plugin_overwrite_comparison and install_theme_overwrite_comparison filters can be used to modify the output of the comparison tables.
• The install_plugin_overwrite_actions and install_theme_overwrite_actions filters can be used to modify the actions displayed after an update has failed when overwriting is allowed.
• The upgrader_overwrote_package action is fired when a plugin or theme has been successfully been upgraded/replaced with a new version uploaded by ZIP.

For more information, refer to #9757 on TracTrac An open source project by Edgewall Software that serves as a bug tracker and project management tool for WordPress..

Themes: functions calling locate_template()now have a return value

The template loading functions have historically not returned any values. When called, they execute silently, even if the desired template file is missing. This makes it difficult to debug situations where there is a typo in the template name details passed to the function.

Starting in 5.5, the following functions now return false if the template file is not found, and void if the template is found and loaded:

• get_header()
• get_footer()
• get_sidebar()
• get_template_part()

This could also be used for themes to check for missing template files and execute code when one is missing. For example:

<?php
if ( false === get_header( 'post' ) ) {
    // Do something when the `header-post.php` file is missing or cannot be loaded.
}

Note: Previously, returning a value when the template was loaded successfully was explored. However, this approach is not feasible. It was discovered that many themes in the wild are printing the return value of get_template_part(), leading to unexpected (and potentially compromising) content being sent to the browser.

For more information, check out the related ticketticket Created for both bug reports and feature development on the bug tracker. on Trac: #40969.

TaxonomyTaxonomy A taxonomy is a way to group things together. In WordPress, some common taxonomies are category, link, tag, or post format. https://codex.wordpress.org/Taxonomies#Default_Taxonomies.: default categories for custom post types

For the post post type, a default categoryCategory The 'category' taxonomy lets you group posts / content together that share a common bond. Categories are pre-defined and broad ranging. term (“Uncategorized” by default) is applied to every published post. Starting in 5.5, this default term can be applied to posts of any custom post typeCustom Post Type WordPress can hold and display many different types of content. A single item of such a content is generally called a post, although post is also a specific post type. Custom Post Types gives your site the ability to have templated posts, to simplify the concept. with the category taxonomy registered to it. This can be accomplished using the new default_category_post_types filterFilter Filters are one of the two types of Hooks https://codex.wordpress.org/Plugin_API/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..

For more information, refer to #43516 on Trac.

Taxonomy: support for default terms in custom taxonomies

A new default_term argument has also been added to register_taxonomy(). Using this argument, a default term name (and optionally slug and description) can be defined. If the defined term does not exist, it will be created automatically.

Additionally, a default_term_$taxonomy option will be added to the database for easy access, and parity with default_category.

For more information, see #43517 on Trac.

Comments: default comment_type value now enforced

Since [9541], an empty string has been used as the default value of a comment’s comment_type field (though it was possible for this to be empty sooner). While examining the possibility of introducing custom comment types in #35214, this was identified as something that would potentially be problematic.

Starting in WordPress 5.5, comment will be enforced as the default comment_type for all comments going forward. This change is one of the first step to open the door to potentially supporting custom comment types.

After upgrading to 5.5, an upgrade routine will be scheduled via cron and run to update all preexisting comments with an empty comment_type to comment. This process will batch comments 100 at a time. The batch size can be adjusted using the wp_update_comment_type_batch_size filter.

If a conditional checks that $comment->comment_type is an empty string in your plugin or theme, it is possible that this change could cause comments to stop displaying on your sitesite (versus network, blog). Twenty Ten was the only default theme affected by this. Though it has been fixed and the theme will be updated along with 5.5, any theme built off of Twenty Ten should be updated.

For more information, see the related tickets on Trac (#49236, #35214).

Comments: Changes to return values

With the aim of consistency and WordPress 5.5 will bring a few changes to the values wp_update_comment() returns.

First, a new parameter, $wp_error, has been added to control whether a WP_Error is returned when an error is encountered. This is optional, and will default to false (do not return a WP_Error object).

Second, the return values for an invalidinvalid A resolution on the bug tracker (and generally common in software development, sometimes also notabug) that indicates the ticket is not a bug, is a support request, or is generally invalid. comment or post ID has been changed from 0 to false. This addresses an issue where 0 could be returned for three different scenarios.

Any code checking explicitly for 0 as a returned value of wp_update_comment() will need to be updated to instead explicitly check for false.

For more information, see the related ticket on Trac (#39732).

Additional developer changes

• Build/Test Tools: The documentation for the Docker environment included with CoreCore Core is the set of software required to run WordPress. The Core Development Team builds WordPress. for local development and the environment overall has been improved. Contributors are encouraged to give it a try and report any breakage they experience. (see #50058).
• Docs: All instances of @staticvar within inline PHPPHP The web scripting language in which WordPress is primarily architected. WordPress requires PHP 5.6.20 docs have been removed. This tagtag A directory in Subversion. WordPress uses tags to store a single snapshot of a version (3.6, 3.6.1, etc.), the common convention of tags in version control systems. (Not to be confused with post tags.) is no longer supported in phpDocumentor (see #50426).
• Formatting: loading has been added as an allowed attribute for the <img> tag in the KSES functions (see #50731).
• Formatting: wp_filter_object_list() now properly returns an array when passed an object with magic methods (see #50095).
• General: The logic in add_magic_quotes() has been adjusted to prevent inappropriately recasting non-string data types to strings. This was noticeable in the REQUEST_TIME and REQUEST_TIME_FLOAT values in the $_SERVER super global (see #48605).
• Login and Registration: The new lostpassword_errors filter in retrieve_password() allows developers to add or modify the errors generated during a password reset request (see #49521).
• Media: the media grid frame is now exposed via the wp-media-grid-ready trigger for easier customization (see #50185).
• Query: A new set_404 action has been introduced, firing after a 404 is triggered (see #48061).
• Query: The found_posts property of WP_Query will now always be an integer (#42469).
• Site Health: The error messages displayed to a user when the site does not meet the PHP or WordPress requirements of the plugin being activated have been improved to list those requirements and include a link to support documentation for updating PHP (see #48245).
• TinyMCE: TinyMCE has been updated to version 4.9.10 (see #50431).
• Upgrade/Install: an additional hook_extra parameter has been added to the upgrader_pre_download filter. This will provide additional context to code hooked onto this filter, such as which plugin or theme update will be downloaded (see #49686).
• Upload: a new pre_wp_unique_filename_file_list filter has been added to wp_unique_filename(). This can be used to short-circuit the scandir() call to provide performance improvements for large directories (see #50587).

Props @desrosj and @sergeybiryukov for reviewing/helping providing some sections.

#5-5, #dev-notes