The bug in here is that you should not be able to add concrete element instances to a display group, and the solution is to make addElement(s) protected instead of public. Any other solution would require a huge overhaul of Zend_Form.

Scheduling for next minor release.

Actually, I was wrong in my previous assessment. We need to limit to only concrete element instances so that the form can add the elements to the display group. However, it was never intended that developers should add elements to the display group manually; only the elements passed to addDisplayGroup() should ever be attached to the display group.

While theoretically we could support this, it would require substantial changes to Zend_Form_DisplayGroup and/or Zend_Form, and for this reason, I'm postponing this issue's resolution.

Matthew, I just ran into this problem as well. Here is the situation I'm in so that maybe you can see a practical use case: An object Foo has a settings form. Foo has an abstract class and can be extended to implement several different types of Foo that use basically the same settings form but the extended versions need to add a couple of fields each. The developer could opt to place the extended fields that customize Foo into their own display groups, but lets say that is not practical and the developer needs the custom fields to be in the same display group as Foo's original fields. So the abstract Foo class sets up a Zend_Form and a display group called "General." Our issue at hand here prevents extended version of Foo from adding fields to "General" very easily.

The work around I think I'm going to use is that rather than having Foo setup the display group directly it will create a hash to organize the fields and loop through them once all the extended fields have been added. That way extended Foo's can add custom fields and simply add them to the same stacks that Foo started. However, I think it would be far less work and less complex if we could simply call $displayGroup->addElement("elementName"); and have it handle all the necessary leg work.

Hope this helps!

I tried to make a patch to Zend_Form like this. Maybe,This relations to ZF-3205..

    public function addElementsToGroup($groupIndex, $elements)
    {
        if (! is_string($groupIndex)) {
            throw new Zend_Form_Exception(__CLASS__ . '::' . __METHOD__ . ' expects a valid DisplayGroup\'s index');
        }
        $group = $this->getDisplayGroup($groupIndex);
        if (! $group) {
            throw new Zend_Form_Exception(' DisplayGroup not found : ' . $groupIndex);
        }
        if (is_string($elements)) {
            $elements = array($elements);
        } elseif (!is_array($elements)) {
            throw new Zend_Form_Exception(__CLASS__ . '::' . __METHOD__ . ' 2nd arg expects string or array');
        }
        foreach ($elements as $element) {
            if (isset($this->_elements[$element])) {
                $add = $this->getElement($element);
                if (null !== $add) {
                    if (array_key_exists($element, $this->_order)) {
                        unset($this->_order[$element]);
                    }
                    $group->addElement($add);
                }
            }
        }
        $this->_orderUpdated = true;
        return $this;
    }

and test it

require_once 'Zend/Form.php';
require_once 'Zend/Form/DisplayGroup.php';
require_once 'Zend/Form/Element/Text.php';
require_once 'Zend/View.php';

$form = new Zend_Form();
$form->setView(new Zend_View());
$form->addElement(new Zend_Form_Element_Text('test'));
$form->addDisplayGroup(array('test'), 'inputs'); // test goes in the display group this way...
$form->setDefaults(array('test' => '1234'));     // and this works fine
var_dump($form->getElement('test')->getValue()); // '1234'

/**
 * How to use this method
 * 1. addElement to form
 * 2. element to group
 */
$form->addElement(new Zend_Form_Element_Text('test2'));
$form->addElementsToGroup('inputs', array('test2'));
$form->setDefaults(array('test' => '1234', 'test2' => '5678')); // test2 is set now, but...

var_dump($form->getElement('test2')->getValue());               // '5678'

echo $form;    // good form.

This code enables to attach elements to displaygroup anytime.