Offense and defense in the world Web_backup

Topic Description: X teacher forget to delete the backup files, he sent Xiaoning students go to find out the backup file, Xiaoning students together to help it!
Solution:
This question is clearly index.php mainly on whether the backup file name format
format suffix .bak general backup file is
therefore accessible URL + / index.php.bak can
use notepad ++ or Notepad to open the file, you can get flag