Standardize sanitization of post title during export

[jmdodd]

Export currently uses apply_filters( 'the_title_rss', $post->post_title ) to sanitize the post title when generating a WXR. This has the side effect of stripping valid HTML tags (reported in #50540) and also creates artificial misses in post_exists tests because newly-encoded characters in the export file do not match those that may already exist in valid titles in the posts table.

An example post title that would have this behavior is: Alice & Bob. This is encoded in the export file as Alice & Bob, resulting in a near-duplicate post on import.

Most other character data in the export is wrapped with wxr_cdata(), and both post content and excerpts have a special export-ready filter:

wxr_cdata( apply_filters( 'the_content_export', $post->post_content ) )

This changeset treats post titles like other character data and provides a filter if additional handlers are needed.

[audrasjb]

Patch refreshed - @since mention added

[audrasjb]

This is a nice enhancement and it also bring a better consistency between the data that are sent to the exporter.

Adding needs-dev-note to make sure it's mentioned into the Miscellaneous Changes dev note.

[pento]

#50540 was marked as a duplicate.

[pento]

Interestingly, this bug does point to the behaviour of the_title_rss being incorrect: per the ​RSS spec, the <title> field can contain HTML tags, clients should just treat it as plain text, though.

I don't think it's worth changing the behaviour of that filter, though. 🙂

[pento]

In 50011:

Export: Create an export-specific filter for post titles.

Since WordPress 2.5 and 2.6, post_content and post_excerpt have both had export-specific filters: the_content_export, and the_excerpt_export, respectively. post_title, however, has used the_title_rss, which behaves differently in two important ways:

• It strips HTML tags from the string.
• It HTML-encodes the title string.

These behaviours are not ideal for exports, since it changes the post title, resulting in data loss in export files, and incorrect post duplicate matching on import. This changes replaces the usage of the_title_rss with a new filter, the_title_export. The new filter is intended to be used in the same as the_content_export and the_excerpt_export.

Props jmdodd, audrasjb.
Fixes #52250.

[audrasjb]

Dev note published: ​https://make.wordpress.org/core/2021/02/23/enhancements-to-the-import-export-feature-in-wordpress-5-7/