I have an established html file website that has a good PR. I have now had the site re-designed with php files & do not wish to lose my hard earned page rank. What can i do to point the searches to the new php files bearing in mind google does not look kindly on sites that contain duplicate content & re-directs?

There was a posting on this not too long ago. You can find some info on it here (http://forums.searchenginewatch.com/showthread.php?t=5907&highlight=htaccess+html+php)

Delete old pages and set up 301 redirects from old URLs to new URLs. Google will find these and pass PR to them.

Keep the same URLs. Parse your html pages as PHP and keep the same URLs--you already have links going to them (internally and from externally) and those URLs are already indexed.

As you create new pages on the site you may want to make them PHP pages, but to be consistent you might as well stick with the .html file extension.