if you have access to your server .htaccess doc (if apache server) the best thing to do is produce a "Redirection".
If the file names are the same between .php and .html this is only a single line (taking into account that they are all in the root directory of your site:
Redirect permanent /(.*).html $1.php
OR you could manualy create a redirect rule for every page: (this is probably the better option!!!!!! but will take you longer to do)
Redirect permanent /page1.html
http://www.yoururlinhere.com/page1.php
Redirect permanent /subfolder/page2.html
http://www.yoururlinhere.com/subfolder/page2.php
.........etc
This means:
Redirect anything ending in {thisword}.html to {thisword}.php
It will help short term but eventually you may drop your pagerank as the new pages are spidered (if they are basically the same content and code...just changed to a php file...you should retain the same
PR but its not gauranteed)
What you are basically doing is telling the SE spiders that the page has been replaced with another permanently. As soon as your have made this file (or updated an existing one!) you can delette your old pages as anything from a spider to a visitor will automatically see your replacement page when trying to access the old one (the server will forward them automatically)
Hope this of help and makes sense.