How to set the value of a field when hidden from certain users/roles?

Question

I have a field which I don't want to be displayed for certain users, however I still want a value to be entered for that field, which is extrapolated from other fields they enter data in.

In order to do this, I'm setting the field as hidden in hook_form_alter:

$form['field_target']['#type'] = 'hidden';

and then setting the value of the field from within a form_validate function based on another field, such as:

$form_state['values']['field_target']['und']['0'] = 
    $form_state['values']['field_source']['und']['0'];

And this works fine, unless I force a form validation error on field_source. When I correct the validation error and submit again, I get a white screen with PHP Error:

PHP Fatal error:  Cannot create references to/from string offsets 
    nor overloaded objects in includes/common.inc on line 6288

This error happens even if I comment out the setting of field_target. However I can fix this error by removing the [#type'] = 'hidden' line and everything works fine, except that the field shows for all users.

Is there any way to fix my above approach, or is there a better way to achieve what I'm trying to do?

Answer 1

Ok it seems I can get around this error by setting:

$form['field_target']['#access'] = FALSE;

instead of ['#type'] = 'hidden'; and Drupal no longer freaks out.

If anybody has a better idea though I'll glady accept a different answer :)