Earlier I made a quick post about php includes and mentioned a couple uses for them. GayStoryMan was kind enough to comment that you can also use them for navigation and that Google does in fact index them. So, here is an updated list of when using PHP includes makes sense:

• Navigation. If you have a site with many pages and navigation running throughout the site, make one quick change to a file and all the pages reflect the change.
• Recip Tables. Have a 100 free sites? Ever find a new link list to submit to? One change to a file and you have all your sites ready to submit to the new link list.
• AEN Sales Pages. I mentioned this before, but it’s both a security net in case the AEN’s change their rules and also allows you to change sales text if you find better sales text. Note: This may not be best if you are creating unique sales text for every site as Google may not like you.
• Sexually Explicit Content Pages: The proposed new rules want the 2257 info on the page and not as a link. This is a great tool to use to change 1000’s of pages in a matter of minutes.

I am sure there are other uses, but these are the ones that I use.