PHP: MySQL request within a function

I want to run a MySQL query within a function in PHP. My code looks something like this, all things, that are not important, I have dropped:

function CheckID($id) {
   $query = "... WHERE id = $id";
   mysql_db_query($db, $query, $con);
}
 
//...
 
// Calling the function causes the error:
CheckID($id); 
 
// The same code from the function at the same place
// works fine:
$query = "... WHERE id = $id";
mysql_db_query($db, $query, $verb);

Now this leads permanently to this error message:

Warning: mysql_db_query() expects parameter 3 to be resource, 
null given in XXX.php on line 16
Warning: mysql_fetch_row() expects parameter 1 to be resource, 
null given in XXX.php on line 17

I have no idea where the bug is. Because, when I place instead of the call of the function, the code of the function at the same place, everything works fine!