A quick footnote:
Other sites (like Wikipedia) tend to dispense with the .php extension. This is (I assume) true, but the odd ".phtml" extension does sneak out from under the covers: this AFAIK is an HTML page with PHP content. It's often asked whether Mediawiki tends to hide most PHP-related extensions.
The answer is this.
The 'other' extensions, such as .php3, .phtml, etc. are all going out, especially because many servers don't parse them. The most universal one is .php, and it is by far the most often used. When you see this in MediaWiki, it's because you're following a "direct" link that isn't being redirected through a rewrite or path info. It is a common thing to avoid use of the .php extension, and more significantly of query strings, when possible because Google tends to treat sites it can flag as "generated content" much worse than those that are static (or generated ones that pretend.) However, this is okay if you don't want the page on Google - for example, this edit page is wiki.phtml?... and no one searches for edit pages, so this is perfect. My point is, it's not that MediaWiki is hiding .php and failing to hide .phtml, but rather that it is just not *always* hiding the .phtml. I would suggest the text is fine as-is.