On a standard LAMP stack, do .php files without any PHP in them get passed to the PHP interpreter?
In other words, is there a performance/processing loss for creating a .php file without actually including any PHP in it, versus just making it a .html file?
Yup – after all, no other component except for the PHP parser is fit to decide whether the file contains PHP!
Potentially, yes, although it will be minimal in most cases unless you have really, really loads of traffic.