How to add a page within another page

Hi,

If you’re referring to rich content like in the visual tab of the post editor, you can add such widget with a plugin like this:
https://www.ads-software.com/plugins/black-studio-tinymce-widget/

Alternatively, you can add a widget that displays a page content dynamically. You can do it with this plugin: https://www.ads-software.com/plugins/query-posts/
You can insert the page ID in the widget and make it show the whole content.

Hope that helps.

Surely there has got to be some way of inserting a page with PHP code?

You can write your own PHP. Or you can use a plugin like Insert Pages to provide a simple shortcode.

So what PHP is necessary to insert the content of a page?