How run a non-WordPress PHP Program on a WordPress site?

Question

Move WordPress from root folder into subdirectory and create a .htaccess file in root folder, and put this content inside (just change example.com and my_subdir):

RewriteEngine on
RewriteCond %{HTTP_HOST} ^(www.)?example.com$
RewriteCond %{REQUEST_URI} !^/my_subdir/
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)$ /my_subdir/$1
RewriteCond %{HTTP_HOST} ^(www.)?example.com$
RewriteRule ^(/)?$ my_subdir/index.php [L]

That’s all 🙂