Like Tom says, all webservers need to know how to handle documents with a particular extention. You will have to tell each and every webserver on this planet how to use *.PHP documents.
Default behaviour for a webserver is to read .html files and copy them 1:1 to the browser.
If you want the webserver to parse .html documents through PHP, you will have to tell the webserver to do so.
What do you mean by "So, php is only scripting "after the fact""?