There is no simple method of overloading constructors in PHP. There are few workarounds to achieve this. I use the following technique.

class UserIdentity extends CUserIdentity
{
	public function __construct()
	{
		$arg_list = func_get_args();
		switch(func_num_args())
		{
			case 1:
				// calling from admin end....
				$this->_user = $arg_list[0];;
				parent::__construct($this->_user->username,$this->_user->password);
			break;
			case 2:
				// calling from Login Model....
				parent::__construct($arg_list[0],$arg_list[1]);
			break;
		}
	}
	// ........
}

 Usage:

// passing two parameters to instantiate the class
$identity=new UserIdentity($this->username,$this->password);
$identity->authenticate();
// passing one parameter to instantiate the same class
$user = Users::model()->notsafe()->findByPk($id);
if(!empty($user)){
	$identity2 = new UserIdentity($user);
	$identity2->loginMember();
}