In this How-To I'll show you:

1.) A reliable alternative to Yii::app()->request->isAjaxRequest (because it doesn't work for everybody and it's not reliable)

2.) How to add extra data to an ajax-request without setting a hiddenField in your form, only using javascript, before sending the form.

Here you can read more about why 'isAjaxRequest' isn't reliable: http://www.yiiframework.com/forum/index.php?/topic/4945-yiiapp-request-isajaxrequest/

The main idea is adding a parameter to our request which tells us if the current request is an ajax-request or not. So if you work with an ajax form, you would for example add a hiddenField named 'isAjaxRequest' and set the value to '1'. But this won't work, if you have a form which can be sent with both, ajax- and usual post-request for users having disabled javascript. So the solution is adding a parameter 'isAjaxRequest' to our form with javascript, before sending it, instead of adding a hiddenField.

After building our form, we add a submitButton like this (in this case for searching users):

<?php 
// Sorry for bad shifting, it's not my fault ...

echo CHtml::ajaxSubmitButton('Search', CHtml::normalizeUrl(array('user/search')), 
     array(
       'data'=>'js:jQuery(this).parents("form").serialize()+"&isAjaxRequest=1"',               
       'success'=>
                  'function(data){
                        $("#searchResult").html(data);
                        $("#searchResult").show();
                        return false;
                   }'    
                                                                                            
     ), 
     array(
        'id'=>'ajaxSubmit', 
        'name'=>'ajaxSubmit'
     )); 
?>  

jQuery(this).parents("form").serialize() is what Yii's ajaxSubmitButton would usually do, if you had not set the 'data' parameter. Now we add +"&isAjaxRequest=1" to set a parameter 'isAjaxRequest' with value '1' to our ajax-request. So this is how to add parameters to ajax-request with javascript. You can add as many parameters as you want like this. js:jQuery(this).parents("form").serialize()+"&isAjaxRequest=1&parameter1=value1&parameter2=value2"

By setting the parameter 'isAjaxRequest' we can filter our ajax request in our controller-action like follows:

public function actionSearch() {
      if($_POST['isAjaxRequest']=='1') {
        // your business logic for ajax-request
        // usually with renderPartial at the end, in order to render a partial view
        $this->renderPartial('ajaxSearchResult', array('param1'=>'value1'), false, true);
      }
      else {
	 // your business logic for non-ajax-request
         // usually with render at the end, in order to render a whole view with your layout
         $this->render('searchResult',array(
		'param1'=>'value1',
	 ));
      }
  }

Very simple, but works ...