Fehler bei der Verbindung zur MySql Datenbank

"; exit; } // else{ // mysqli_select_db($mysqldb, $con); // $database = mysqli_select_db($con, $mysqldb); // } ?>
Warning: Undefined variable $con in /data/web/e40454/html/m4c/php/sub2.php on line 65

Fatal error: Uncaught TypeError: mysqli_query(): Argument #1 ($mysql) must be of type mysqli, null given in /data/web/e40454/html/m4c/php/sub2.php:65 Stack trace: #0 /data/web/e40454/html/m4c/php/sub2.php(65): mysqli_query() #1 {main} thrown in /data/web/e40454/html/m4c/php/sub2.php on line 65