Solve PHP/mysqli problem + provide working source code

here's my problem :

I created a simple php user registration + login system...
everything works fine... the user can register and log in + form data is stored in database... after login the user lands on index.php, which is the main user profile... I got it to work so that it shows the username that logged in on that index, "welcome user123" ... now I want other database data of that user to be displayed on that user's index.php too, e.g. registration date, or any other database fields I choose... I tried to echo the data just like the username is echoed but it doesn't work for some reason...

I need you to find the mistake and fix it, so I also can display other user database data of the logged-in user on the users index.php / profile... and also please show me where I was wrong so I learn it for the future... I can provide you with all php code(s) ... my codes are all really very basic since I'm a php beginner ... so If you're experienced with php and mysqli it should probably take you about 10-15 minutes to fix my problem ...