Modifying .htaccess to interpret .HTML files in the filesystem as PHP

I am using MODx in Revolution 2.5. I have files in the filesystem that are .html, but I would like to add PHP code to these files and have it interpreted. So far, the only way that I have been able to get the PHP code interpreted is to rename the files to .PHP. For other reasons, I would like to keep the files .html.

I've tried to work with the .htaccess file by adding "AddHandler application/x-httpd-php .php .html". I've added this at the bottom of the file in the root with no success. I've added new .htaccess files (e.g. in the same directory as the .html files) and modified the ht.access file as well. Still, the .html files do not get interpreted as PHP files.

Is there a way to get the .html files to be interpreted as PHP so that the PHP code can run?
How can I get changes to the .htaccess file to be reflected in MODX Revolution 2.5?