I am thinking of changing my pages so they have php headers and footers and sidebar, but my index page as a page rank of 2, so if I change my page and put in the php pages for the header, footer and sidebar, would it wrong to leave it .html or do I have change it to .php
You can still leave pages with PHP in as HTML versions (I believe). If this is not the case, your page rank will stay if you change the old html page to a 301 redirect.
If all you intend is to provide a single source file for each of these sections, you need not use php include() functions. The Apache server, and others, provide for a server-side include. Normally, you'd change the .html extension to .shtml to process the instruction. There is an alternative, though; the XBitHack. You leave the extension as .html but set the execute bit on the file. See Apache Tutorial: Introduction to Server Side Includes http://httpd.apache.org/docs/2.2/howto/ssi.html and XBitHack Directive. http://httpd.apache.org/docs/2.2/mod/mod_include.html#xbithack cheers, gary