Drupal: Pitfalls when converting CCK field modules from Drupal 5 to Drupal 6

I recently needed to convert a CCK field module from Drupal 5 to Drupal 6. There is a lot to take in, but I was faced with a particular problem:

warning: array_shift() [function.array-shift]: The argument should be an array in D:\wamp\www\drupal6\includes\form.inc on line 1320.

Oh dear. It's always difficult to debug this kind of thing because the problem lies in the code that called the function on line 1320 of form.inc, rather than there being a problem with form.inc. I used debug_backtrace() to see what parameters the previous functions in the call stack were using, and noticed that only the first two parameters in _form_set_value() were populated; the other two were NULL. This was the immediate source of the error. The following line was failing because $parents was NULL:

$parent = array_shift($parents);

Obviously you can't array_shift() NULL. I then went back further in the backtrace, to the form_set_value() function (note the function is not preceded by an underscore, like the last one). In this function, the second parameter ($value) was NULL. This was causing the NULL in the _form_set_value() function.

The solution

It turns out that I was using this code to handle the form widget's processing in my CCK field module:

/**
* Process the mcimage element.
*/
function mcimage_mcimage_process($element, $edit, $form_state, $form) {
 $field_name = $element['#field_name'];
 $field = $form['#field_info'][$field_name];
 $field_key = $element['#columns'][0];
 $value = isset($element['#value'][$field_key]) ? $element['#value'][$field_key] : '';

 $element[$field_key] = array(
   '#type'           => 'hidden',
   '#default_value'  => $value,
   // The following values were set by the content module and need
   // to be passed down to the nested element.
   '#title' => $element['#title'],
   '#description' => $element['#description'],
   '#required' => $element['#required'],
   '#field_name' => $element['#field_name'],
   '#type_name' => $element['#type_name'],
   '#delta' => $element['#delta'],
   '#columns' => $element['#columns'],
 );
}

My mistake? The function does not return anything! It was as simple as adding a return statement at the end of the function, so that the form element could be processed correctly:

/**
 * Process the mcimage element.
 */
function mcimage_mcimage_process($element, $edit, $form_state, $form) {
  $field_name = $element['#field_name'];
  $field = $form['#field_info'][$field_name];
  $field_key = $element['#columns'][0];
  $value = isset($element['#value'][$field_key]) ? $element['#value'][$field_key] : '';
  
  $element[$field_key] = array(
    '#type'           => 'hidden',
    '#default_value'  => $value,
    // The following values were set by the content module and need
    // to be passed down to the nested element.
    '#title' => $element['#title'],
    '#description' => $element['#description'],
    '#required' => $element['#required'],
    '#field_name' => $element['#field_name'],
    '#type_name' => $element['#type_name'],
    '#delta' => $element['#delta'],
    '#columns' => $element['#columns'],
  );
  
  return $element;
}