Forum Moderators: Robert Charlton & goodroi
Other than that, Google doesn't care about the file extension, whether .php, .html, .aspx or anything else. Only the HTML output is parsed, not the underlying code.
Even dynamic URLs (with variables) are no longer a problem for Google, as long as they are set up so as not to create infinite duplicate content or "infinite content loops" (for example where dynamic URLs can be incremented to infinity). So the potential weakness of dynamic sites are in relation to their relative complexity, and not in relation to their underlying technology.
I'd also recommend always using the .php extension.
I'd recommend that you rid yourself of the extensions altogether. Just don't use them anymore. They serve no purpose in the overall scheme of things other than to cause grief for Webmasters when they need to change the underlying technology of the site.