Iv'e tried everything, I enabled rewrite module, messed around with the apache config and still my server will not add the .php extension automatically. It just keeps showing a 404 error.

BTW, I have this setup on a virtual host.

Here's my Apache config:

<VirtualHost *:80>
        # The ServerName directive sets the request scheme, hostname and port th$
        # the server uses to identify itself. This is used when creating
        # redirection URLs. In the context of virtual hosts, the ServerName
        # specifies what hostname must appear in the request's Host: header to
        # match this virtual host. For the default virtual host (this file) this
        # value is not decisive as it is used as a last resort host regardless.
        # However, you must set it for any further virtual host explicitly.
        #ServerName www.example.com

        ServerAdmin webmaster@localhost
        DocumentRoot /var/www/ce

        ServerName computingessentials.tk
        ServerAlias www.computingessentials.tk

        # Available loglevels: trace8, ..., trace1, debug, info, notice, warn,
        # error, crit, alert, emerg.
        # It is also possible to configure the loglevel for particular
        # modules, e.g.
        #LogLevel info ssl:warn

        ErrorLog ${APACHE_LOG_DIR}/error.log
        CustomLog ${APACHE_LOG_DIR}/access.log combined
         <Directory /var/www/ce/>
                Options Indexes FollowSymLinks MultiViews
                AllowOverride All
                Require all granted
         </Directory>
        # For most configuration files from conf-available/, which are
        # enabled or disabled at a global level, it is possible to
        # include a line for only one particular virtual host. For example the
        # following line enables the CGI configuration for this host only
        # after it has been globally disabled with "a2disconf".
        #Include conf-available/serve-cgi-bin.conf
</VirtualHost>

And here is my htaccess

   RewriteEngine on

   RewriteRule ^([0-9a-zA-Z-]+)$ $1.php [NC,L]
   RewriteRule ^([0-9a-zA-Z-]+)/$ $1.php [NC,L]

   RewriteRule ^videos/([0-9a-zA-Z-]+)$ videos.php?var=$1 [NC,L]
   RewriteRule ^article/([0-9a-zA-Z-]+)$ article.php?var=$1 [NC,L]
   RewriteRule ^video/([0-9a-zA-Z-]+)$ video.php?var=$1 [NC,L]

For example, there is a page called series.php, which should get replaced to /series but when I go to /series, it says file not found.

And you will not be able to access the server since at the moment I have setup my host files to redirect me to the IP.