HTML files saved with a PHP extention

Not necessarily. The file will function just the same with a PHP extension as it would with a regular HTML extension. One thing I like about automatically creating all files with PHP extensions is that if later you want to add something in PHP to the file, you don't have to save the file again with the new extension or worry about changing any Apache settings.

Saving static files as PHP puts an extra load on the server, because it still loads the PHP engine to parse it.

However, as Kurtis said, I would agree that if there is a change of including PHP in the future, it may be best to do it that way.

As previously stated, having a plain HTML file parsed for possible PHP content will introduce a bit of 'unnecessary' load.

If you are worried about that difference, why not do a benchmark between the two to see if the overhead is significant enough for you to continue to worry?

Cheers,
Shaggy.

If you might add PHP code in the files in the future, as has already been mentioned, then I think it's a very good idea.
As usually, when you add PHP to one file, you add it to many as the PHP usually requires various files to function and also offers functionality to multiple files or site wide.

Also, if you link to a page(s) with .html, then change it (them) later to .php you have to change all links to that page(s) throughout your site.

Also, if you link to a page(s) with .html, then change it (them) later to .php you have to change all links to that page(s) throughout your site.

A little rewrite magic could fix that:

That is true, and there is also a way to make all HTML files parse PHP:
AddType application/x-httpd-php .php .htm .html 
(this will vary slightly depending on server software)

Although there are downsides to both methods:
Mine makes ALL files are parsed even if they are simple plain old html, which adds server load for no reason in HTML only files.

And your method you have to add a line for every page you want to parse.

In summary, there are ways around it, but it's better practice to use .php for php files and then .html for html only.

I always use a header and footer file included in pretty much all my sites, as well as always using some other PHP even if just for simple little functionality. so I always use .php anyway.

And your method you have to add a line for every page you want to parse.

Are you talking about the .htaccess code?