How to solve the Undefined MYSQLI and Uncaught TypeError: MySQLI_Query()

Please help me to solve this,

Warning: Undefined variable $mysqli in
C:xampphtdocssyslaporanauthauth.php on line 10

Fatal error: Uncaught TypeError: mysqli_query(): Argument #1 ($mysql)
must be of type mysqli, null given in
C:xampphtdocssyslaporanauthauth.php:10 Stack trace: #0
C:xampphtdocssyslaporanauthauth.php(10): mysqli_query(NULL,
‘SELECT * FROM t…’) #1 {main} thrown in
C:xampphtdocssyslaporanauthauth.php on line 10

I just update my XAMPP to the new one, and use PHP 8 right now. I just started to learning php and start with configure my login page.

Koneksi.php

Auth.php

Give me advice, Give me some codes to solve my problem