<?php
// $Id: mymodule.module


/**
 * Implementation of hook_menu().
 */
function mymodule_menu() {
  $items = array();

    $items['mymodule-change-opstina'] = array(
    'page callback'    => 'mymodule_change_opstina',
    'access callback'  => TRUE,
    'type'             => MENU_CALLBACK,
	'access arguments' => array('access content'),
  );

  return $items;
}


/**
* Implementation of hook_form_alter().
*/
function mymodule_form_alter(&$form, $form_state, $form_id) {

  switch ($form_id) {
    case 'my_content_type_node_form':
		$form['#cache'] = TRUE;
		
		//if it is already set, like on user/1/edit page
		if(isset($form['field_category'][0]['#default_value']['value']) && $form['field_category'][0]['#default_value']['value']!="")
			$category_default_value =$form['field_category'][0]['#default_value']['value'];
		else
			$category_default_value = 1; //my first item in vocabulary
		$options1 = array();
		$category_array = taxonomy_get_tree(1, 0, -1, 1); // 1= my vocabulary ID, 0=parent, -1=depth, 1=max_dept
		foreach($category_array as $value) 
			$options1[$value->tid] = ($value->description?$value->description:$value->name);
		
		//first dummy field that collect value for cck filed filed_category
		$form['category'] = array(
			'#type' => 'select',
			'#title' => t('Category'),
			'#options' => $options1,
			'#default_value' => $category_default_value,
			'#required' => 1,
			'#ahah' => array(
			  'event'   => 'change',
			  'path'    => 'mymodule-change-opstina',
			  'wrapper' => 'edit-subcategory-wrapper', //here enter id of a wrapper that contains your field
			  'method' => 'replace',
			),
		  );
	
		//if it is already set, like on user/1/edit page
		if(isset($form['field_subcategory'][0]['#default_value']['value']) && $form['field_subcategory'][0]['#default_value']['value']!="")
			$subcategory_default_value =$form['field_subcategory'][0]['#default_value']['value'];
		else
			$subcategory_default_value = 7; //my first item under the item with id 1 ,in vocabulary
		
		$options2 = array();
		$subcategory_array = taxonomy_get_tree(1, $category_default_value, 0, 2); // 1=vocabulary ID, =parent, 0=depth, 2=max_dept
		foreach($subcategory_array as $value) 
				$options2[$value->tid] = ($value->description?$value->description:$value->name);		
	
		//second dummy field that collect value for cck filed filed_subcategory
		$form['subcategory'] = array(
			'#type' => 'select',
			'#title' => t('Subcategory'),
			'#options' => $options2,
			'#default_value' => $subcategory_default_value,
			'#required' => 1,
		);
		
		//this is needed for javascript to enter values in real cck fields
		$form['buttons']['submit']['#attributes'] =  array('class' => 'submit-insert');
		
		break;
	}
}

function mymodule_change_opstina() {
	// The form is generated in an include file which we need to include manually.
	include_once 'modules/node/node.pages.inc';
 
	// We're starting in step #3, preparing for #4.
	$form_state = array('storage' => NULL, 'submitted' => FALSE);
	$form_build_id = $_POST['form_build_id'];
 
	// Step #4.
	$form = form_get_cache($form_build_id, $form_state);

	// Preparing for #5.
	$args = $form['#parameters'];
	$form_id = array_shift($args);
	$form_id = $args;
	$form_state['post'] = $form['#post'] = $_POST;
	$form['#programmed'] = $form['#redirect'] = FALSE;

	// Step #5.
	//drupal_process_form($form_id, $form, $form_state);

	// Step #6 and #7 and #8.
	// $form = drupal_rebuild_form($form_id, $form_state, $args, $form_build_id);

	//change your element here
	//here we are changing list of opstions for subcategory, considering the choosen value for category
	$options3 = array();
	$category_id = $_POST['category']; 
	$subcategory_array = taxonomy_get_tree(1, $category_id, 0, 2); // 1=vocabulary ID, =parent , 0=depth, 2=max_dept
	foreach($subcategory_array as $value) 
		$options3[$value->tid] = ($value->description?$value->description:$value->name);
	$form['subcategory']['#options'] =  $options3;

	form_set_cache($form_build_id, $form, $form_state);

	$form += array(
	'#post' => $_POST,
	'#programmed' => FALSE,
	);

	$form = form_builder('my_content_type_node_form', $form, $form_state);

	// Step #9.
	$subform = $form['subcategory'];

	unset($subform['#prefix'], $subform['#suffix']);
	$output = theme('status_messages') . drupal_render($subform);

	// Final rendering callback.
	drupal_json(array('status' => TRUE, 'data' => $output));
	}
?>

<script type="text/javascript">
  Drupal.taxonomy_initialize = function() {
  	
	$(".submit-insert").click(function(){
		if ( $('#edit-category').length > 0 ) {
			var cat = $('#edit-category').val(); 
			$('#edit-field-category-0-value').val(cat);
		}
		if ( $('#edit-subcategory').length > 0 ) {
			var subcat = $('#edit-subcategory').val(); 
			$('#edit-field-subcategory-0-value').val(subcat);
		}
	})
}

if (Drupal.jsEnabled) {
  $(document).ready(function(){
	Drupal.taxonomy_initialize();
  })
}
</script>