Part of my website is static, and others will be .php. For the static pages, I want to also save them as .php files. Does it make any difference from save as .htm files? Will this affect the downloading speed for visitors while viewing my static pages?
Yes, but the difference won't be noticeable. By default, PHP parses all files with an .php extension before it sends the output to the browser. That means it checks for <?php tags and parses the content of them. (If any). While .html files won't be parsed by default. So they're served directly. But this is really nothing you should worry about. The difference is so small, no one would ever notice.
If your pages are just plain html there is no reason to work with PHP. It can't provide anything for you but slow down your web site due to the extra resources it takes due to the php interpreter. Web servers can serve a significant amount of static pages compared to interpreted language pages I'm just not sure how much of an effect just running the interpreter itself will do on server resources without it actually doing any real logic.
The reason I want to save as php is that I can use <? include header.php; ?> to share the same header and footer for all the pages. I don't like using the html templates. As frankcow said, php is very good for templates. So if I save the .htm files as .php, the only codes will be the <? include header.php; ?>. Will this affect too much on the speed? Or are there any other good choices to share the same header or footer in .htm files? BTW, I have only around 15 pages for my website.
You shouldn't notice any difference in speed just using php to include a header and footer. If in doubt - test it out and see - I bet you won't notice any difference at all.
There is a big difference between php and html. It's not the same type, so you can't actualy ask .htm or .php.
the only difference between the two is that one is sent through the parser, and one is not. it's a matter of milliseconds, you'd never notice the difference.