redirect all .html extensions to .php

You could do a more simple approach and have all your html files be processed as php files by adding the following line to your .htaccess

AddHandler application/x-httpd-php .php .html

RewriteEngine On
RewriteRule ^(.*)\.html$ $1.php [L]

If you want it to be done as a redirect instead of just a rewrite modify the [L] to [L,R]