水野史土

This plugin diplays a is_page()/is_category()/is_tag() snippet for page/category/tag list. You can go to the page/category/tag list in the admin area, and you can copy one (and paste it to a theme file).

php snippet for theme designers

Samuel Wood (Otto)

The normal Text widget allows you to insert arbitrary Text and/or HTML code. This allows that too, but also parses any PHP code in the text widget and executes it.
This can make it easier to migrate to a widget-based theme. However, this plugin should not be used long term, as anybody with access to edit the widgets on your site will be able to execute arbitrary PHP code.
All PHP code must be enclosed in the standard php opening and closing tags ( <code><?php</code> and <code>?></code> ) for it to be recognized and executed.
Only users with the unfiltered_html role will be allowed to insert unfiltered HTML. This includes PHP code, so users without admin or editor permissions will not be able to use this to execute code, even if they have widget editing permissions.

PHP Code Widget

Simply Awesome !!! 

WHY IS THIS NOT IN THE CORE, AND WITH THE ADDED FUNCTIONALITY TO EXECUTE PHP IN THE EDITOR???

No, instead we waste our time with useless Gutenberg !!!

Thanks for the plugin, it is a time saver for me. I can put the code in a widget, and display the widget in the content area via a shortcode... does the job well.

Unfrieking Unfreikingbelievable !!!

No highlighting for snippets.Support also doesn't exists. You can find much better plugins to insert php code snippets

much better solutions

Like the Text widget, but also allows working PHP code to be inserted.

php snippet for theme designers vs PHP Code Widget