In response to Sun Love, the code you posted works well except for situations where you have a trailing slash without the file extension (I get a 500 error) because the first RegEx doesn't match for this situation.
example.com/test.html -- works (redirects to /test)
example.com/test -- works (no redirect)
example.com/test.html -- works (redirects to /test)
example.com/test/ -- doesn't work (500 error)
There is probably a better way to do this but I added another rewrite condition to fix this:
RewriteEngine on
Options +FollowSymLinks -MultiViews
RewriteBase /
## hide .php extension
# To externally redirect /dir/foo.php to /dir/foo
RewriteCond %{THE_REQUEST} ^[A-Z]{3,}\s([^.]+).php
RewriteRule ^ %1 [R=301,L]
RewriteCond %{THE_REQUEST} ^[A-Z]{3,}\s([^.]+)/\s
RewriteRule ^ %1 [R=301,L]
## To internally redirect /dir/foo to /dir/foo.php
RewriteCond %{REQUEST_FILENAME}.php -f
RewriteRule ^(.*)$ $1.php [L]