Options -MultiViews
RewriteEngine On
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)/$ $1.php [L]
RewriteCond %{REQUEST_FILENAME}.php -f
RewriteCond %{REQUEST_FILENAME}.html -f
RewriteRule ^(.*)/$ $1.html [L]

Options -MultiViews
RewriteEngine On

RewriteCond %{REQUEST_FILENAME} -d [OR]
RewriteCond %{REQUEST_FILENAME} -f
RewriteRule ^ - [NC]

RewriteCond %{REQUEST_URI} ^/(.+?)/$
RewriteCond %{DOCUMENT_ROOT}/%1.html -f
RewriteRule ^ /%1.html [L]

RewriteCond %{REQUEST_URI} ^/(.+?)/$
RewriteCond %{DOCUMENT_ROOT}/%1.php -f
RewriteRule ^ /%1.php [L]