I would use .inc
(meaning include file) or .txt
.
I generally use .inc
(include file) or .shtml
(shared HTML). I've never heard of .pht
, though.
As the files are really written in Hyper Text Mark-up Language, I think it's quite valid to give them the .html extension. Consider just calling the Directory something like "divs" or "panels", "forms", etc...
I've only seen the ".phtml" extension before but I've never used it. I always use ".html", like any other ".html" file. Like MasterPeter said the file is written in HTML, so I think it's correct.
Another extension, ".fhtml", appears to be used by some outdated Macromedia product, and at least one other software package uses it to mean "fragment HTML".
Having a precedent is nice, but unfortunately the .fhtml suffix also used to indicate HTML templates with embedded Factor code. That kind of ambiguity is annoying.
I use "inc". I don't see any reason to worry about a thing like that. Call it whatever you want, as long as you get the benefit of looking at the files in a directory listing and not being confused.
One example where I don't use "inc" would be when there are includes inside includes, say your outer page includes a nav include, which then includes a third file. In that case you have to stay with whatever your server expects the extension to be for server-parsed files.