How much does having the php extension hurt your SEO effort?

Hi, i've been reading around on these forums for a little while, and the stuff i've seen has really helped me alot with getting my google rankings up a bit. I have a question though, how much does having the .php extension hurt your pagerank? In the case of my site, I use it so that I can include header and footer files, plus so I can compress my pages dynamically. I don't have any variables passed on load however.

I was also thinking that if it would help me out to remove the php extension, what is the best extension to replace it with? I know that .html is a good choice, but I was also thinking that I could use xhtml to show off that I use w3c compliant xhtml (I also use their buttons on my site).

I have another question: is it detrimental to put your forums on a subdomain, like forums.foo.org (where the main site is foo.org)? I am doing this at my site so that I can remove my forum traffic from my main sites logs, plus visitors think its cool.

Oh yes, your SEO mod rocks Craven!!!!