Apache/PHP not displaying .php documents correctly

Here's the problem:

Just upgraded Apache & PHP on my server. Some .php pages are being served correctly, though others are requesting to be downloaded. I'm not sure what the problem is, and it seems to work fine in IE, yet Mozilla wants to download the file, full source and all.

Another odd thing, http://<sitename>.com/ will attempt to download the file, where http://<sitename>.com/index.php will view the file in the browser as it should.

Edit:
When viewing <sitename>.com/ from mozilla here's what I get:
---------------------------
You have chosen to open

from: http://<sitename>.com/
which is a:
---------------------------

Very confusing if you ask me. Any help would be gladly appreciated.

Many thanks.

Look at your Apache config-file. There is a line where you can set which pages apache will search for as the default document. Probably there now is something like:
index.html index.htm
When you add index.php you solved at least one part of your problem. I don't recognize the other thing though, but maybe it's solved when you add the line in your Apache-config.

Sounds like your mime types aren't defined correctly.