If I change it to php will it affect seo on html website?

Selected answers from the Dumb SEO Questions Facebook & G+ community.

• 
  Pankaj Giri: Yes! If URL will be change in .php it will hamper a short of, but if you will start a genuine SEO & Back Link building again for your website, That will gain the rank and traffic with in your time frame :)
• 
  Dave Elliott: on page stuff no. Link building, yes. But you maybe be able to just write a urlrewriter rule so that anything that comes into the site as .html accesses a .php  ;page of the same name
• 
  iqdesign: If you change that's the problem but if it is possible to maintain both old and new url until your divert all your traffic to new url then it's not a problem as I think. I am not a professional. ;
• 
  Pankaj Giri: It is not mandatory to have .php extension for php script to run, One can easily modify the extension through .htaccess (means you can keep your extension as it is) and some php framework like codeIgnitor, cakePHP also allow you to choose any extension regardless of back end technology.!!
• 
  Justin Y: The simplest is probably like everyone has mentioned by creating a mod rewrite rule.
  
  The example below should do the trick but you'll want to test it first because I don't know if you need a query string added to it or not.
  
  Options +Indexes
  Options +FollowSymlinks
  RewriteEngine on
  RewriteBase /
  RewriteRule ^(.*).html$ $1.php [L]
• 
  Andrei Gherghel: I agree with +Justin Y