This plugin shows post/page/category/tag/comment/media/user IDs on admin's edit post/page/category/tag/comment/media/user pages. You can see the ID when you visit an edit post/page/category/tag/comment/media/user page.
Since this plugin hooks on 'edit' link, only users with priviledges can see IDs. For example, a user of author role can see post IDs, while a user of subscriber role cannot.
You can find a brief description in Japanese on a book
Introduction to PHP for WordPress Users. (written in Japanese.)
2.5
fix for notice when in trash.
2.4
span class="wp-ui-text-primary" added. For more readability.
2.3
indent and spacing fix
2.2
add prefix for function name
2.1
update donation link
2.0
media/user IDs are shown.
IDs are shown without rollover.
1.0
post/page/category/tag/comment IDs are shown.