Some people want to show .html or .php in the url of their wordprss site. For SEO these are very handy, you can make your website url far more search engine friendly.

Solution:
Go to the the wp-admin/ admin panel of your wordpress site. Click settings, then permalink settings, then click custom structure. Now in the input box.

If you want to show .html in your url, type this
/%postname%.html

and if you want to show .php type this
/%postname%.php

Hope this might help you to make your websie url SEO friendly.