This is an easy to maintain method for creating clean urls without any file extensions. It's very good for hiding your underlying programming language and offers a more user friendly experience. It only uses a single series of mod_rewrite rules then all the rest of the logic can be accomplished within your script. You'll need mod_rewrite enabled and you can put these rules into your .htaccess or settings in httpd.conf.

RewriteEngine on
RewriteCond %{DOCUMENT_ROOT}%{REQUEST_URI} !-f
RewriteCond %{DOCUMENT_ROOT}%{REQUEST_URI} !-d
RewriteRule ^.*\?{0,1}(.*)$ /index.php?$1 [QSA,L]

Then within your PHP code you can handle the REQUEST_URI to determine what page to display. You can use query strings normally by accessing the $_REQUEST array.

$uri = $_SERVER[REQUEST_URI];

if(preg_match("/\/aboutus\//", $uri)) {
    include('about_us.php');
}
else if(preg_match("/\/category\/([^\/]+)\//", $uri, $matches)) {
    // Ex: /category/php/ or /category/java/
    $category = $matches[1];
    include('category.php');
}