RewriteEngine On
#example.com/page will display the contents of example.com/page.html
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME}.php -f
RewriteRule ^(.+)$ $1.php [L,QSA]

#301 from example.com/page.html to example.com/page
RewriteCond %{THE_REQUEST} ^[A-Z]{3,9}\ /.*\.php\ HTTP/
RewriteRule ^(.*)\.php$ /$1 [R=301,L]