[Andreas]

Can you give the exact location where this is stated in the manual?
As it seems to be wrong

You cannot create methods for objects at runtime; this is a PHP limitation.

What you can do is using a lamda function that calls your custom verification function:

PHP Code:

if (!function_exists('verify_fieldname'))
{
    function verify_fieldname(&$dm, &$data)
    {
        /*<do stuff>*/
    }
}


$this->validfields['fieldname'] = array(TYPE_HERE, REQ_NO, 'return verify_fieldname($data, $dm);');