php doesn't work on all directories

Question

I installed lamp server and changed it's directory to my Dropbox folder following all the instructions here.

The first page I opened was in http://localhost/askRond/Website/ and it opened without any problem.

But when I created index.php in .../Dropbox directory and put some php code inside the file, nothing was displayed when I opened localhost/ in the browser. The source code of the page was completely empty.

When I moved the index.php to another file test the same problem appeared. But when I moved it to askRond/Website/ file all php in the file was executed successfully.

The apache2 file content

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

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

<Directory /home/nullsky/Dropbox/>
    Options Indexes FollowSymLinks
    AllowOverride None
    Require all granted
</Directory>

#<Directory /srv/>
#   Options Indexes FollowSymLinks
#   AllowOverride None
#   Require all granted
#</Directory>

The Dropbox.conf content without comments(based tutorial, I made a new .conf file instead of 000-default.conf

<VirtualHost *:80>

    ServerAdmin webmaster@localhost
    DocumentRoot /home/nullsky/Dropbox

    ErrorLog ${APACHE_LOG_DIR}/error.log
    CustomLog ${APACHE_LOG_DIR}/access.log combined

</VirtualHost>

I searched for this problem on the internet but could not found a solution.

How can I solve this problem?