PHP set field-name with parameter
Solution 1:
You have to whitelist the values of $field and concatenate it into the query:
if (in_array($field, array('allowed_field_1', 'allowed_field_2'))) {
$sql = "UPDATE STA_Kunden SET " . $field . " = ? WHERE (KUN_KundeNo = ?)";
$params = array($value, $select);
}