Any existing backlinks to your pages would now be broken, you could use a 301 redirect sending visitors to the old address to the php counterpart, this redirect can be done in an .htaccess file, maybe a line per page, maybe, a smarter script can redirect all html to php.

If Google has already indexed your old urls, there will be many 404 error. You need to use 301 redirect from old url to new. If you cannot do that remove manually old links from Google and submit new sitemap.

Yes - you need to build 301 redirects with .htaccess from old links to new links.

If you don't do this - your site will be crawled with 404 errors and your ranking will be hurt.

301 Redirection is the solution .

Now i am re-constructing a website, from Joomla to PHP (pure PHP light weight site). The Urls path are same except ending with php. it means previous www.url.com/page is migrated as www.url.com/page.php is it impact SEO ranking? How to prevent it?