Add basic, extensible sitemaps.

[adamsilverstein]

This ticket tracks work towards integrating basic XML Sitemaps functionality into WordPress Core.

For reference, sitemaps for core was ​originally proposed in June 2019 and a feature plugin was created. An ​update was posted in January. Weekly, open team meetings started in ​February (​meeting recaps). The feature plugin is also ​available on the .org repository for testing.

Development work is wrapping up on the feature plugin ​in GitHub where feedback and testing are welcome. The team will soon publish a merge proposal as outlined in the ​Handbook.

Work is ​underway now to prepare the feature plugin codebase for a core merge; once that is complete we will post a patch here.

Merges the Sitemaps feature plugin.

Repository: ​https://github.com/GoogleChromeLabs/wp-sitemaps

Background: ​https://make.wordpress.org/core/2019/06/12/xml-sitemaps-feature-project-proposal/

Trac ticket: https://core.trac.wordpress.org/ticket/50117

[whyisjake]

Pumped to see the pull request here!

[swissspidy]

#12301 was marked as a duplicate.

[jonoaldersonwp]

Added #50400, to update the 'Search Engine Visibility' site settings to mention their impact on XML sitemaps. Would be nice to ship these together.

[whyisjake]

PR looks great, let's get this added!

[swissspidy]

In 48072:

Sitemaps: Add XML sitemaps functionality to WordPress.

While web crawlers are able to discover pages from links within the site and from other sites, XML sitemaps supplement this approach by allowing crawlers to quickly and comprehensively identify all URLs included in the sitemap and learn other signals about those URLs using the associated metadata.

See ​https://make.wordpress.org/core/2020/06/10/merge-announcement-extensible-core-sitemaps/ for more details.

This feature exposes the sitemap index via /wp-sitemap.xml and exposes a variety of new filters and hooks for developers to modify the behavior. Users can disable sitemaps completely by turning off search engine visibility in WordPress admin.

This change also introduces a new esc_xml() function to escape strings for output in XML, as well as XML support to wp_kses_normalize_entities().

Props Adrian McShane, afragen, adamsilverstein, casiepa, flixos90, garrett-eclipse, joemcgill, kburgoine, kraftbj, milana_cap, pacifika, pbiron, pfefferle, Ruxandra Gradina, swissspidy, szepeviktor, tangrufus, tweetythierry.
Fixes #50117.
See #3670. See #19998.

Shipped! https://core.trac.wordpress.org/changeset/48072

[swissspidy]

In 48079:

Sitemaps: Bump $wp_db_version to flush rewrite rules after [48072].

Props ocean90.
See #50117.

[SergeyBiryukov]

In 48080:

Coding Standards: Remove unused variable in WP_Sitemaps_Provider::get_sitemap_entries().

Follow-up to [48072].

See #49542, #50117.

[SergeyBiryukov]

In 48081:

Coding Standards: Fix WPCS issues in wp-includes/sitemaps/.

Some of these were not caught when running composer lint:errors, but are fixed when running phpcbf directly.

Follow-up to [48072], [48080].

See #49542, #50117.

[SergeyBiryukov]

In 48094:

Sitemaps: Rename the wp_sitemaps_is_enabled filter to wp_sitemaps_enabled.

This makes it more consistent with the other filters of the similar purpose.

Props arpitgshah.
Fixes #50428. See #50117.

[johnbillion]

In 48098:

Docs: Corrections and improvements to inline docs related to XML sitemaps.

See #50117, #49572

[justinahinon]

Dev note link: ​https://make.wordpress.org/core/2020/07/22/new-xml-sitemaps-functionality-in-wordpress-5-5/