How to aquire a page file name by using page id

Hi i need to set up a system that allows me to re-direct users that are logged in to a different page than those who are not. Iv been able to do this (as below) but xxx.php may be renamed (by other people) and I dont want to have to keep comming back and edditing the template so is there anyway I can use the page ID to GET the file name of the page?

i.e. $WB_ID=20->$file_name;

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Working code:

if(FRONTEND_LOGIN AND $wb->is_authenticated()) {
header('Location: '.WB_URL.'/pages/xxx1.php');
}elseif{
header('Location: '.WB_URL.'/pages/xxx2.php');
}

Thanks!