I have to admit, I'm partially writing this post just to see in my analytics reporting how many people are searching for it. But for those that are trying to wedge PHP into Drupal8 and having trouble with it, there is something here for you.
The PHP Filter has been moved out of core and is now a contib module. It was also not working during the release-candidate phase of Drupal 8, but seems to be back on track now. Sometimes people like to do crazy things like create a Panels page at a certain URL and then enable the PHP text filter. If you're slick you might do an include() or an include_once() command and call your scripts from a directory where they can be managed in a code repository and such.
But it's a drag creating a huge Panels page, with all its features, just to call an include() command. And you have to create a Panel for each path (or route) you want to bring into existence. This is pretty heavy for just having a route to call a script.
The Slipscripts module is very straightforward and does just that: It allows you to define a verb (or route) and an attached PHP script to run at that route. So you can have routes like http://MySite.corp/discount/20 and feed the "discount" and
"20" variables to a script to do some calculations and export the result. The entire thing is done in a "Drupal-friendly" way that respects Drupal8's routing and general MVC-style concepts.
I am pushing that module into a sandbox today and will post back here when it is available. Enjoy!