Need Help - I am trying to make multiple database conncetion based on user

[56]

I want to make a multiple database connection based on new users that register in system. Logic is like this: I have a function that connect with main database below:
function db_connect() {
  static $connection;
  if(!isset($connection)) {
    $connection = mysqli_connect('localhost','main_db_user','main_db_pass','main_db_name');

  if($connection === false){ return mysqli_connect_error(); }
  return $connection;
}
I have created this function inside a file called "conn.php". So, I have created a list with some databases and I want to connect each of them for each new user that is registered in system. In some details: How to change 
the connection(to connect with a new database that I have created before) when a new user is registered in the main database?

[511]

you're one of the few people that formatted a high quality question, so thank you for that. Not a web developer this seems like a better question on stackoverflow since no one has responded here.

[56]

this is a bump