How to let PHP to create subdomain automatically for each user?

Question

You’re looking to create a custom A record.

I’m pretty sure that you can use wildcards when specifying A records which would let you do something like this:

*.mywebsite.example       IN  A       127.0.0.1

127.0.0.1 would be the IP address of your webserver. The method of actually adding the record will depend on your host.

Then you need to configure your web-server to serve all subdomains.

Regarding .htaccess, you don’t really need any rewrite rules. The HTTP_HOST header is available in PHP as well, so you can get it already, like

$username = strtok($_SERVER['HTTP_HOST'], ".");

If you don’t have access to DNS/web-server config, doing it like http://mywebsite.example/user would be a lot easier to set up if it’s an option.