technically this breaks MVC separation. but it can be handy (for formating data being stored to a database from an action, for example. This is most easily accomplished by copying the use_helper code to a custom class.

create a custom class in project\lib (or app\lib). i'll use Misc.class.php.

<?php
 
class Misc
{
    public static function use_helper($helperName)
    {
      static $loaded = array();
      if (isset($loaded[$helperName]))
      {
        return;
      }
 
      if (is_readable(sfConfig::get('sf_symfony_lib_dir').'/helper/'.$helperName.'Helper.php'))
      {
        // global helper
        include_once(sfConfig::get('sf_symfony_lib_dir').'/helper/'.$helperName.'Helper.php');
      }
      else if (is_readable(sfConfig::get('sf_app_module_dir').'/'.sfContext::getInstance()->getModuleName().'/'.sfConfig::get('sf_app_module_lib_dir_name').'/helper/'.$helperName.'Helper.php'))
      {
        // current module helper
        include_once(sfConfig::get('sf_app_module_dir').'/'.sfContext::getInstance()->getModuleName().'/'.sfConfig::get('sf_app_module_lib_dir_name').'/helper/'.$helperName.'Helper.php');
      }
      else
      {
        // helper in include_path
        include_once('helper/'.$helperName.'Helper.php');
      }
      $loaded[$helperName] = true;
    }
}

(clear your cache after adding a custom class, of course).

now to use a helper from an action:

Misc::use_helper('Date');
....