How to use .htaccess to run all .html files as .php by using PHP handlers in Apache w/o CGI?

If you are not able to find the correct Handler, Simply create a .php file with the following contents:

<?php echo $_SERVER['REDIRECT_HANDLER']; ?>

and run/open this file in browser.

Use this output in .htaccess file

Create a .htaccess file at the root of your website(usually a folder named public_html or htdocs on linux servers) and add this line:

AddType [[THE OUTPUT FROM ABOVE FILE]] .html .htm

Example

AddType application/x-httpd-php70 .html .htm
AddType application/x-httpd-php72 .html .htm
AddType application/x-httpd-php .html .htm

If your are running PHP as CGI then try something like this:

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

Important Note:

If you see blank page or Notice: Undefined index: REDIRECT_HANDLER

Try default in .htaccess

AddHandler application/x-httpd-php .html

If you are godaddy user, try one of these settings:

Options +ExecCGI
AddType application/x-httpd-php .php .html
AddHandler x-httpd-php5 .php .html
AddHandler x-httpd-php7 .php .html
AddHandler x-httpd-php5-cgi .html
AddHandler x-httpd-php7-cgi .html

Reference: https://stackoverflow.com/questions/4687208/using-htaccess-to-make-all-html-pages-to-run-as-php-files/49375772#49375772