Hello Any one know the rules for htaccess,

I have follow requirement:

http://www.domainname.com/aboutus.php--input
http://www.domainname.com/aboutus/--Need this output

RewriteEngine on

RewriteCond %{HTTP_HOST} ^www.yoursitename.com [NC]
rewritebase /

RewriteRule ^$ index.php [L]
RewriteRule ^index index.php [L]
RewriteRule ^aboutus  aboutus.php [L]

last post save as .htaccess and next define below code in config page

$Site_URL = "http://yoursitename/";

define('SITE_URL',$Site_URL);

call now page like this

<a href="<?=SITE_URL?>aboutus">aboutus</a>