variable as DB-fieldname

When I call a form I pass some parameters via url. One of them is the fieldname of a database where I want to store a number.
Is it possible in CF6 to use a variable as fieldname?

Hi Bournell,
​
The calling URL variables are added to the form data by ChronoForms - if you temporarily add a Debugger action you will see them there. Please see this FAQ for the corresponding short-codes.
​
Bob

I don't think you can use a variable to replace the field name, but it would take you mere seconds to try.

I spent nights to find it out - no chance.
BUT
I want to use a php-block with this code:

$start =  $_GET["zeit"]; // These are the parameter I pass tu the Form via url
$position = $_GET["pos"];
$team  = $_GET["team_nr"];

$db =\JFactory::getDbo();

    $query = $db->getQuery(true);

    $query -> update  ('#__3s_starts') // this is my table where
    -> set  ("$position=$team")       // I want to put the team-number
    -> where ('Zeit='.$start);       // at this specific time.
 
$db->setQuery($query);

echo $query->dump(); //for testing purpose

$db->close();

The Query-Dump :  

UPDATE svt38_3s_starts
SET Position1=63
WHERE Zeit=1567927800

But it does not work inside CF. Nothing will be written into the DB. ( It's possible to read from DB.) Pleeeze give me a hint....

OK so I don't see there where you actually EXECUTE the query?