Help With A php script

[COAD]

$f = $HTTP_GET_VARS['f'];
$label = $f;
$f = str_replace(" ", "", $f);

$wide = $HTTP_GET_VARS['wide'];
if ($f == "") { 
	$f = "Home1";
}

if ( (substr_count($f,"..")!=0) || (substr_count($f,"http")!=0) ) {
	$f = "Error";
}
$ext=".php";
    if(!is_readable($f.$ext))
    {
      $ext=".php";
      if(!is_readable($f.$ext))
      {
        $f="error";
        $ext=".php"; 
      }
    }

then I just

//i put this where i want the page.
include($f.$ext);

If you didnt notice, the function of this script is to include a page into the index file without putting the graphics over and over again. (EX: http://domain.com/index.php?f=file)

File=the php/html file.

My questioin is, how would I do a subcategory?