redirect all .html extensions to .php

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]

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

mod_rewrite to the rescue!

RewriteEngine On
RewriteRule ^(.+)\.html$ $1.php

In your apache httpd.conf file you can add

AddType application/x-httpd-php .html

to make .html files go through the php parser before they are served to the user. You can also add this directive to your .htaccess file. The second method may not work depending on how your host is setup.