should be something like
PHP Code:
<?
$activeadmlist = "user_list.txt";
$current = file_get_contents($activeadmlist);
$pos = strpos($current, $_SESSION['username']);
if ($pos === false)
{
$current .= $_SESSION['username']."\n";
file_put_contents($activeadmlist, $current);
}
else
{
//TODO:: do someting
}
?>
used the code flow from your duplicate post
untested, but should work