Re: What mysql user is put into mysql_connect() for a registration page?
In the mysql server, add the desired user to mysql using phpmyadmin or some other mysql tool. Then grant the desired permissions for that user on the database your application will be using. Refer to the GRANT statement documenbtation in mysql to see the sql used.
Then in your code identify the mysql user account to use in your connection. Yes, typically such information is in a configuration file for the php application.