Accessing a View function in a helper. The current view object is stored in the _View attribute of the Helper class is extended by AppHelper and all other Helpers in CakePHP. $this->assign is a view function and if you want to use this view function in a Helper this is how we use it :

MyHelper.php

<?php
App::uses('AppHelper', 'View/Helper');
class MyHelper extends AppHelper{
        public function assignTitle($pageTitle){
            return $this->_View->assign('pageTitle', $pageTitle);
        }
}
?>