Try this code in your DOCUMENT_ROOT/.htaccess
file:
RewriteEngine On
# see if .php file can be found with same name
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{DOCUMENT_ROOT}/$1.php -f
RewriteRule ^(.+?)/?$ /$1.php [L]
# now see if .html file can be found with same name
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{DOCUMENT_ROOT}/$1.html -f
RewriteRule ^(.+?)/?$ /$1.html [L]