How do I set up wildcard *DOMAIN* on my droplet?

@Cybil Here is the approach I would take:

Update DNS to direct all subdomain requests to your Droplet

1. Add your domain webapp.com without any subdomains to the DO Control Panel.
2. Add an A Record entry to DNS where an asterisk * is the host name and your droplet is selected as the target so that all subdomains route

Now the DNS record is directing all subdomain URL requests (user1.webapp.com, user2.webapp.com) to your Droplet. To ensure the root domain is also pointing to webapp.com, add an @ A-Record too.

Note: From our documentation:

(Use) A wildcard (*). To direct requests for a non-existent subdomain to a server or load balancer, enter *. However, if any kind of DNS record exists for a hostname, the wildcard will not apply; you will need to explicitly create an A record for it. https://www.digitalocean.com/docs/networking/dns/how-to/manage-records/#a-records

Update Apache config on Droplet to map all subdomains to the right webapp route

1. Enable the mod_vhost_alias extension on Apache with sudo a2enmod vhost_alias and then restart Apache with systemctl restart apache2
2. Update your Apache config to add a VirtualHost block that maps subdomains to the proper PHP route

# ROOT DOMAIN CONFIG
<VirtualHost *:80>
    ServerName webapp.com
    DocumentRoot /path/to/webapp
</VirtualHost>

#DYNAMIC SUBDOMAIN CONFIG
<VirtualHost *:80>
    ServerAlias *.webapp.com
    VirtualDocumentRoot /path/to/webapp/users/%1.php
</VirtualHost>

Lastly restart apache again with systemctl restart apache2