RewriteEngine On
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule ^([^.]+)$ $1.php [NC,L]

Examples:
index.php?id=xx will be index?id=xx
index.php?page=xx will be index?page=xx
users.php?uid=xx will be users?uid=xx
news.php?nid=xx will be news?nid=xx

RewriteEngine On
RewriteCond %{THE_REQUEST} s/+(.+).php[s?] [NC]
RewriteRule ^ /%1 [R=302,L,NE]

RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^([^.]+)$ $1.php [NC,QSA,L]