redirect - Page navigation with php -

i trying create dynamic website, index.php includes following code in content area of website:

<?php  // if undefined define  if(!$od || $od == ""){  $od = "news";  } // check if exists prep  $link = 'incs'."/".$od.$ext;  flush();  $fp = fopen($link, "r");  // check if inclusion file not exists return error  if (!$fp) {  echo "an error has ocurred while processing request. file linked ?od=".$od." not appear exist.";  }  // if exists return parse  else {   fclose($fp);  include 'incs'."/".$od.$ext;  }  echo '</body>'."\n"; echo '</html>'."\n";  ?> 

i have various links throughout site pages register, login, etc. links point pages ?od=register, ?od=login, etc.

the website pull default file me, news, , display in content section of website, when click register, url in address bar change /?od=register, default news remains in content section, there error in code above? or missing something?

p.s. $ext defined in config file inc.php, included @ top of index page.

besides fact insecure - making request, access variables through $_get array ie $od = $_get['od']