OverTheWire Natas 21

Code analysis shows that we need the 'admin' field to be set to 1 in order to get the next level credentials.
There is also another page in this level - http://natas21-experimenter.natas.labs.overthewire.org/ The code of this page contains the following block:

if(array_key_exists("submit", $_REQUEST)) { 
    foreach($_REQUEST as $key => $val) { 
    $_SESSION[$key] = $val; 
    } 
} 

If 'submit' was sent as a GET parameter, each (key,val) tuple in the GET request will become a key in the session with 'val' as value.
We will use it to set the session key-value ('admin',1).

The session is shared across the pages, therefore when we will back to http://natas21.natas.labs.overthewire.org/ , we will be identified as admin ! Solved :)