With a few years of Drupal 6 and 7 experience, I've happily made my first Drupal 8 installation. In my previous installations of Drupal I've used custom made PHP code that does a lookup in a personal database. I still need this in Drupal 8.
I've managed to install the PHP filter, and scripts are executed fine, but the page becomes cached, and changes in my database are not seen before I edit the page or empty the cache manually. Inconvenient, to say the least.
According to documentation, PHP filtered pages will not be cached, but evidently my page is.