Level 15 in phoenix ,firewall

I had participated in an event that is organized by DCE named firewall it is over now
in this event on last level 'We are given login box in order to authenticate "
with no password and username
so i begin with finding hidden pages with robots.txt file there i find file that contains username and password and there was also a .php file where i have to put that authentication information but when i opened that .php
there was nothing ...
when i asked the admin to give some clue then he say "You have to reach server side of that file" I don't understand what he mean?
Is there anyone here who can help me ?I want to solve that level