When you have your site accessible both under your_domain.com and www.your_domain.com you may come up with duplicate content on Google Search. To avoid such problems you can use the following lines in your .htaccess file to force only the www version of your web site:
RewriteEngine on
RewriteCond %{HTTP_HOST} !^www.your_domain.com$
RewriteRule ^(.*)$ http://www.your_domain.com/$1 [R=301]
Note that the .htaccess should be located in the web site main folder.
This will redirect all requests to the non-www version of your site to the www version using 301 Permanent redirect which will make the search engines to index your site only using the www.your_domain.com URL. In this way you will avoid a duplicate content penalty.
This similar to the www vs non-www version of your site work-around.
As you might now you, by default you can access your site as http://www.domain.com/ and http://www.domain.com/index.html with some setups it can be index.php or index.asp or default.aps, etc.
Unfortunately, this creates a risk of duplicate content from the search engine point of view. To avoid this, you can use the following ModRewrite rules in your .htaccess file:
RewriteEngine on
RewriteCond %{THE_REQUEST} ^[A-Z]{3,9}\ /index\.html\ HTTP/
RewriteRule ^index\.html$ http://www.domain.com/ [R=301,L]
The file should be located in your website main folder (web root).
The rules above will tell the browser or the search engine both that all requests to index.html should be directed to the "/" (slash only).
By default most web servers across the internet are configured to treat as PHP files only files that end with .php. In case you need to have your HTML files parsed as PHP (e.g .html) or even if you want to take it further and make your PHP files look like ASP, you can do the following:
For web servers using PHP as apache module:
AddType application/x-httpd-php .html .htm
For web servers running PHP as CGI:
AddHandler application/x-httpd-php .html .htm
In case you wish to do the ASP mimick:
For PHP as module:
AddType application/x-httpd-php .asp
OR for PHP as CGI:
AddHandler application/x-httpd-php .asp