Hi..
My site was first developed in HTML and i optimized it well and keywords ranking is also in top ten in google.com but after converting whole site in PHP it's ranking goes invisible and in google webmaster toll it's showing 404 error for HTML pages but i removed it also but no effect occur .....Anyone have any suggestion please

As your previous pages are indexed with .html extension and now they are not their your ranking is Going down.
Install a sitemap.xml file in root directory of your server and Submit it at Google webmaster central.

Before Google indexed and traffics .html page but right now you had converted .php so .html page removed from Google server, .PHP page will crawl Google then you will be getting traffics..

good content is very important of course - but don't forget about keywords that will help you to increase your PR too. Use right tool to find them - i can recommend semrush.

For making index frequency for new page redirect 301 from html pages.