It\’s probably not that expensive since there probably aren\’t any PHP tags in the static .html files. But it\’s still a cost. PHP startup is not free.

It might be better to use mod_rewrite to target individual URLs. This way, if you receive a request for your well_known_page.html, mod_rewrite will rewrite it to call the .php instead. Alternatively, you could have your .html redirect to the .php (you get the benefit of the good rating, plus, possibly, the search engine will start to give the .php a good rating too). I\’d probably go with mod_rewrite though.