Add Condition In Yii Search

Hi,

I have a list view populated using cgridview. Outside the grid view i have a seach form where various parameters can be entered. On clicking submit form, i should be able to get the searched result using the values provided.

I have succeeded in filtering the results using single variable. But when multiple values are to be searched i cannot make it to work.

Controller :

$this -> layout = 'main';
if(isset($_GET['search_key'])){
$search = $_GET['search_key']; 	
$model->firstName = $search;  
$model->lastName = $search;  
$model->email = $search;
}      	
$this -> render('staffmanagement', array('model' => $model));

Model :

public function search()
	{
		// Warning: Please modify the following code to remove attributes that
		// should not be searched.

		$criteria=new CDbCriteria;

		$criteria->compare('title',$this->title,true);		
		$criteria->compare('firstName',$this->firstName,true,'OR');
		$criteria->compare('lastName',$this->lastName,true,'OR');
		$criteria->compare('email',$this->email,true,"OR");
		$criteria->compare('userType',$this->userType,true);
		$criteria->compare('mob_organisationId',$this->mob_organisationId,true);
		$criteria->compare('createdDate',$this->createdDate,true);
		$criteria->compare('modifiedDate',$this->modifiedDate,true);
		$criteria->compare('active',$this->active,true);

        $criteria->addCondition('firstName',$this->firstName,'OR');
        $criteria->addCondition('lastName',$this->lastName,'OR');
        $criteria->addCondition('email',$this->email,'OR');

		return new CActiveDataProvider($this, array(
			'criteria'=>$criteria,
		));
	}

View :

<?php 
echo CHtml::beginForm('', 'get'); 
echo(CHtml::textField('search_key', '',array('class' => 'search_txt', 'placeholder' =>'Search')));
echo CHtml::endForm(); 
?>

When i remove all the addCondition() in model

        $criteria->addCondition('firstName',$this->firstName,'OR');
        $criteria->addCondition('lastName',$this->lastName,'OR');
        $criteria->addCondition('email',$this->email,'OR');

as well as remove two values from controller

$model->lastName = $lastName;  
$model->email = $email;

the search works perfectly for a single value (firstName).

How can i add other two parameters also and keep the form working ?