"how to let php to create subdomain automatically for each user?" Code Answer

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.com       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.

Doing it like http://mywebsite.com/user would be a lot easier to set up if it's an option.

Then you could just add a .htaccess file that looks like this:

Options +FollowSymLinks

RewriteEngine On
RewriteRule ^([aA-zZ])$  dostuff.php?username=$1

In the above, usernames are limited to the characters a-z

The rewrite rule for grabbing the subdomain would look like this:

RewriteCond %{HTTP_HOST} ^(^.*).mywebsite.com
RewriteRule (.*)  dostuff.php?username=%1

However, 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'], ".");