I recommend people don't do this unless their web server is going
to serve only(or mostly) php pages, and very few straight html pages.
....the increase in overhead isn't really worth it.
I've tried this in many real-world examples.
I have PHP parsing ALL files on ALL of my 8 Linux servers.
(I do virtual hosting for 500 or so domains.)
Even though they're running on weak little Cobalt RaQ boxes, there has been no measurable slowdown since I switched Apache to having PHP parse all files.
I know, in THEORY, it slows it down. I haven't done deep deep benchmarks. But just real-world experience has been great. No slowdown at all.