<?php
function redirectToLogin($msg = 'You must login first')
{
    global $mainframe;
    $currentUser =& JFactory::getUser();

    //If user is not logged-in
    if($currentUser->get('guest'))
    {
        $uri = JFactory::getURI();
        $return  = $uri->toString();

        $url = 'index.php?option=com_user&view=login';
        $url .= '&return='.base64_encode($return);

        $mainframe->redirect($url, JText::_($msg));
        exit();
    }
}
?>