I am trying to create a PHP page with my Drupal 8 components but I only found a documentation for module examples.
The objective is to generate a PHP page with the header, menu, and footer from my Drupal 8 and then, insert my PHP code in this page.
Here the Drupal documentation I think you will find your happiness
You cannot get drupal header, footer and menu separately on any php page but if you want then do a simple approach.
1 - Generate header, footer and menu content in text or html file(this to be done from cms)
2 - Include those file in the php file
Thanks