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index.php gets no respect?

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I've had a search around the board but can't find any specific debate on this one. Since my php knowledge is somewhat sketchy, I'll throw it to the experts...

Take a popular php site in any particular area. High rankings and high Google PR for www.samplephpfoosite.com.

1. Does the PR carry over to www.samplephpfoosite.com/index.htm - even though that file doesn't exist? (Looks like it in some cases.)
2. Does the PR carry over to www.samplephpfoosite.com/index.php (which might exist)? (Looks like it doesn't in some cases.)
3. If it does, is it due to the internal linking? (E.g. linking to index.php rather than to the main URL.)
4. Does changing the default page to index.php in .htaccess have any effect?
5. Does it matter anyway?