Being a simple soul I design my sites in html. I have a customer who has "bought" a mobile version of his site, Effectively this is a .mobi domain name and code to convert hispages. He's sent it through to me to implement for him.
Here's the nub - it requires all his pages to be in php and so I need to convert all his pages from html to php.
Here's the question - will changing index.html to index.php, prices.html to prices.php affect his search rankings?
Technically, .php is a different page than .html and you'll lose some link weight through the redirects, so imo it would be better to rewrite the php to the html extensions, or parse the html as php. You can find out more about how in the Apache Forum [webmasterworld.com]. It's actually fairly simple to parse the html as php if it's going to be every page.
There is no need to change the URLs when you add PHP functionality to a site.
There are two ways to do this.
Either force .html files to be parsed for PHP content, or change the files from .html to .php and implement an internal rewrite so that when a .html URL is requested the server looks for the matching .php file to carry out the request.
To be clear, URLs and files are not at all the same thing, and the action of a server is to map a URL request to an internal file.