Yes, it is and unless you have a compelling reason not to adhere to that practice I strongly recommend that you name PHP files .php (or, actually anything but .html!) Older php programs used to be named .php3, to distinguish them from PHP4 programs, but that created more than enough confusion for new programmers.
Please do not take this as being condscending or disrespectful of your knowledge, but the kinds of things you can do to have .html files parsed as PHP by PHP, seem to me to be a little advanced for you, given that you were not aware that the parser for PHP operates on .php files. There are ways to do this, and it is not hard to set it up, using .htaccess directives, but I submit you will be opening yourself up to the very real possiblility of major confusion if you attempt to operate that way before you have a better understanding of what’s happening.
If you really want to persue that, please become familiar with all the ramifications of doing so, and research the proper way to do it. See this link , and look near the bottom of the page for “How can I use ssi on files with .html extensions?”, for the general idea, then research how that would apply if you substitute php for ssi - same concept, you define a custom handler.
You can also search this forum, or the web in general, for more information on that - I’d really just rather not encourage you to do that until you fully understand it, as it could considerably complicate your learning. It is hard enough for experienced php programmers to keep straight, at times, what is being parsed as php and what is parsed as html within a single .php file. Preparing pages and finding errors when “everything is parsed as php”, even in an .html file, requires a very methodical process and is not advised while learning PHP. When the time comes where you can identify a reason to do it, you will probably have learned enough PHP to know how it’s done
edit: While I was posting this, Atropos7 pointed you to the specifics, which really was a much more direct answer to your last question. I was just trying caution you - what Atropos7 wrote is right on the money!