How To Set Up Subdomain or multi-domains and Apache Virtual Hosts on Ubuntu 14.04 LTS

gistfile1.txt

Summary
=========
1) Create an A record for your sub-domain that points to your droplet
2) Create a .config file for your sub-domain site in /etc/apache2/sites-available
3) Create the directory for your site in /var/www (or wherever you said it was in the config file)
4) Enable the site in apache with: sudo a2ensite sub.yourdomain.com (or whatever you named your site in the config)
5) reload apache: sudo service apache2 reload
NOTE: the DNS record you create might take awhile to propagate
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1). 
In domain's DNS settings (e.g. digitalocean)
A--->demo.example.com.--->IPAddress (NOTE: there's a "." at the end of demo.example.com)

If using CDN (e.g. cloudflare)
A--->demo---------------->IPAddress [DNS & HTTP proxy(CDN)]

2). 
/* make directory for a new subdomain or domain under the same directory as the original website or create one if not exists. */
Examples:
sudo mkdir -p /var/www/example.com/
sudo mkdir -p /var/www/demo-site/ (would become demo.example.com)
sudo mkdir -p /var/www/example2.com/

/* grant permission to the new directory: */
sudo chown -R $USER:$USER /var/www/test.com/

/* modify permissions to ensure that read access is permitted: */
sudo chmod -R 755 /var/www

/* Copy the default Virtual Host and Customize for Second(sub/new) Domain */
clone existing 000-default.conf under /etc/apache2/sites-available/
sudo cp /etc/apache2/sites-available/000-default.conf /etc/apache2/sites-available/000-demo.conf

/* The new config file must be modifed something like below */
sudo nano 000-demo.conf    

<VirtualHost *:80>
        <Directory /var/www/demo>
                Options Indexes FollowSymLinks MultiViews
                AllowOverride All
                Order allow,deny
                allow from all
        </Directory>
        ServerName demo.example.com
        ServerAlias www.demo.example.com
        ServerAdmin webmaster@localhost
        DocumentRoot /var/www/demo
        ...
</VirtualHost>

NOTE: forgetting to do the step above will cause database or cache conflict. 

after that, enable the New Virtual Host Files as below:
E.g:
sudo a2ensite example.com.conf
sudo a2ensite 000-demo.conf

then, reload apache
sudo service apache2 reload


Ref: How to Set Up Apache Virtual Hosts on Ubuntu 14.04 LTS
https://www.digitalocean.com/community/tutorials/how-to-set-up-apache-virtual-hosts-on-ubuntu-14-04-lts

TKS Bro, you save my life today!