failed to remove index.php. lamp on ubuntu 14.04

Yes, finally I fixed this problem.

The solution:

1. open /etc/apache2/apache2.conf;
2. find the following lines:
  <Directory /var/www/>

          Options Indexes FollowSymLinks
          AllowOverride None

  </Directory>
3. change "AllowOverride None" to "AllowOverride All"
4. restart apache2

Then everything goes smoothly.