Pretty simple, but you'll need to handle the coding part.

• Create an A record at DO for a wild-card sub-domain (*.users.your-domain.com, for example)
• In apache, create a wild-card vhost:

Example vhost config (written for CentOS 6.6)

<VirtualHost *:80>
        ServerName users.your-domain.com
        ServerAlias *.users.your-domain.com
        #### This is where you put your files for that domain: /var/www/vhosts/c.jonsjava.com
        DocumentRoot /var/www/vhosts/users.your-domain.com/
        <Directory /var/www/vhosts/users.your-domain.com>
                # This section sets directives for the directory.
                # -Indexes <-- That blocks index listings on folders that don't have a default file (index.php/index.html/default.html/etc)
                # FollowSymLinks <-- This will treat symlinks like they should be treated in linux: as folders/files in the folder the symlink resides
                # MultiViews  <--It's easier for you to read this: http://httpd.apache.org/docs/2.0/content-negotiation.html#multiviews
                Options -Indexes FollowSymLinks MultiViews
                # AllowOverride All <-- This is required for Apache HTTPD server to read .htaccess files.  It says that you can have a per-folder override for apache directives
                AllowOverride All
        </Directory>
        ## Everything to see here. Just the log files.  Good to use for troubleshooting errors.
        CustomLog /var/log/httpd/users.your-domain.com-access.log combined
        ErrorLog /var/log/httpd/users.your-domain.com-error.log

        # Possible values include: debug, info, notice, warn, error, crit,
        # alert, emerg.
        LogLevel warn
</VirtualHost>

• In your PHP, have it get the host name:

$URL = $_SERVER[SERVER_NAME];
$user= str_replace(".users.your-domain.com","",$URL);

Now, you have the user. Just have your code configured to show that users page, or a 404 if it doesn't exist.