I cannot provide a php file for download within my article.
I installed https://wordpress.org/plugins/wp-extra-file-types/# and disabled all uplaod checks. I Uplaoded my php. I enabled all checks again. The URl from the Media Library, does still not download, but executes/opens.
My Solution to this was finally, placing the .php file inside of a zip File, which is upload-/ and downloadable by default.