My apache2 installed on the debian8.1.
In the configuration file /etc/apache2/apache2.conf

<Directory />
Options FollowSymLinks
AllowOverride None
Require all denied
</Directory>

<Directory /usr/share>
AllowOverride None
Require all granted
</Directory>

<Directory /var/www/>
Options Indexes FollowSymLinks
AllowOverride None
Require all granted
</Directory>

There are lines in the index.html:

The default Debian document root is /var/www/html. You can make your own virtual hosts under /var/www. This is different to previous releases which provides better security out of the box.
With phpinfo() to get document root is /var/www.

How to change it into /var/www ?In the /etc/apache2/apache2.conf,it is already <Directory /var/www/>, but in fact the document root is /var/www/html.