how to set default document on apache

Set apache to accept it:
sudo gedit /etc/apache2/sites-available/default



Options Indexes FollowSymLinks MultiViews
AllowOverride All
Order allow,deny
allow from all



On your app i.e. /var/www/app create .htaccess file with this:

Order deny,allow
ErrorDocument 400 error.php
DirectoryIndex list.php