Dynamic form fields, dynamic form field validations: how to validate?

[eluser]CodeIgniterNewbie[/eluser]
I have an application that allows users to create forms and assign validation to each field (think http://www.phpform.org/ or http://wufoo.com/). I can easily get each field's name and label from the database, as well as the array of associated validation rules.

Say, for example, I wanted to apply a blanket rule of "required" for all of the user defined forms, I would do something like this:

foreach($fields as $field)
{
$this->form_validation->set_rules($field['name'], $field['label'], 'required');
}

Now, the problem is that I need to replace "required" with a callback. In order for the callback to work, I'm going to need the field's id. How do I get this id value to the callback function?

[eluser]LuckyFella73[/eluser]

In order for the callback to work, I’m going to need the field’s id.

Why do you need ids of the form elements for you callback function?
The callback function runs server side - how can the id be usefull for validating?

In case I'm wrong, the only way to build these rules would be sticking to naming conventions
for on server side you can't read ids from parsed html sourcecode that does not
exist when the callback function is called.