При запуске программы php я застрял в этой ошибке

Warning: Undefined variable $con in C:\xampp\htdocs\shop\admin\category information.php on line 7

Fatal error: Uncaught TypeError: mysqli_query(): Argument #1 ($mysql) must be of type mysqli, null given in C:\xampp\htdocs\shop\admin\category information.php:7 Stack trace: #0 C:\xampp\htdocs\shop\admin\category information.php(7): mysqli_query(NULL, 'select max(Cate...') #1 C:\xampp\htdocs\shop\admin\category information.php(36): makeID('categoryinforma...', 'Category_ID', 'CAT-', '7') #2 {main} thrown in C:\xampp\htdocs\shop\admin\category information.php on line 7

И код показан ниже

        $query=mysqli_query($con,"select max($field) from $table");
        $fetch_max=mysqli_fetch_row($query);
        

А код подключения к базе данных приведен ниже

$con=mysqli_connect("localhost","root","","onlineshoping");
$select_db=mysqli_select_db($con,"onlineshoping");