[=OTS=G-Man]

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