title: How to modify .htaccess file in order to redirect some html pages if they don't exists?
body: <p>I want to redirect all the .html file like this:</p>
  
  <p>www.sitename.com/art/5.html</p>
  
  <p>to the following (equivalent) php page:</p>
  
  <p>www.sitename.com?art=5</p>
  
  <p>but ONLY if the .html file doesn't exist, otherwise it should not redirect. What should I write in my .htaccess file?</p>