the login session on non-WHMCS pages?

hello

i try to create a whmcs login menu on non-WHMCS page for my wordpress site, i put the following codes in my theme function, and create " if ($_SESSION['uid'])...else " codes on my homepage, it is working, but when i go to my wordpress appearance/menu/ and try to add menu or editing different menu, the editing page goes blank; i create a non-encrypted php file with some dummy text and put it in whmcs folder, and set the same path as following in my theme function, the editing page appear again. any helps?? thanks

require($_SERVER['DOCUMENT_ROOT']."/whmcs/dbconnect.php");
require($_SERVER['DOCUMENT_ROOT']."/whmcs/includes/functions.php");
require($_SERVER['DOCUMENT_ROOT']."/whmcs/includes/clientareafunctions.php");

function user_get_id()
{
$pagetitle = '';
$pageicon = "";
$breadcrumbnav = '';
initialiseClientArea($pagetitle,$pageicon,$breadcr umbnav);
$sessionid = $_SESSION['uid'];
return $sessionid;
}