Is there a way to allow PHP in the posts themselves?
Do you mean executing PHP code?
What do you want to achieve by that?
I'd like to include an RSS feed as part of the post
Do you mean executing PHP code? What do you want to achieve by that?
In the 2000' it was a major concept, there were dedicated application servers often for enterprises. With the advent of powerful clients and subsequent Javascript rise (including XMLHttpRequest client-server communication) this had slowly been forgotten.
https://en.wikipedia.org/wiki/Application_server https://en.wikipedia.org/wiki/Server-side_scripting
You can modify the post.html.php and include your PHP code there.