To begin with, there's an include function in PHP:

The include (or require) statement takes all the text/code/markup that exists in the specified file and copies it into the file that uses the include statement. https://www.w3schools.com/php/php_includes.asp

I have an HTML form with an action link and once I submit the form, it redirects me to that target page.

What I'd like to do is instead of redirecting me to that target page, the contents of the target page will load on the same page as the form like how include works. It won't be a new page, it will still be the same but the contents of the target page will be added into it. How would I go about this?