Which is the best way to explain this I wonder?
On most servers, if it receives a request for index.html (or index.htm) the server tries to find the file, picks it up and serves up the contents of that file.
On a PHP configured server, if it receives a request for index.php the server tries to find the file, picks it up, passes it to PHP which does some work and then serves up the contents of that file.
index.php might contain a mixture of straight html but also some PHP - eg <?php echo time(); ?>
That is great too.
Now, what you want is to have your server serve up a html file -- BUT you want it to be passed to the PHP processor.
Whoa. So now you want to break the general rule, so be fully aware that what you want to do goes against the grain.
Can it be done ***, certainly, yes, as has been explained previously by the good folk of this forum.
Does that take some knowledge, yes it does.
Is this exception to the rule documented in PHP books, no it is not.
Is that the fault of the PHP book authors? No, not in my opinion, nor in the opinion of others on this forum.
Let me try and explain it with a simple allegory.
Imagine you bought a normal little compact car from me, and then decide you would like to cross the Russian tundra in winter in my little car.
You are going to need at least snow chains for your tyres, and extra strong anti-freeze so your engine does not freeze up.
Just because you cannot find any reference to snow chains and how to handle -70 degree temperatures in the owners hand-book does not make me, nor does it make the car -- nor even the owners handbook -- invalid.
It is doable.
It means you have to accept that what you want to do is outside of the norm, so you will have to do some work, but don't go phoning me from Moscow saying your car broke down.
*** I ran a large site which was written in .asp, so I reconfigured the server to serve up .php pages as .asp pages so that we would not lose any SEO points and left the URLs as they were. I'm not saying I would do the same now, but it is doable.