$app = 'frontend' ;

$currentApp = sfConfig::get('sf_app') ;
sfConfig::set('foo', 'bar') ;
  
sfContext::getInstance()->getLogger()->info('Foo = ' . sfConfig::get('foo') . ' in ' . $currentApp) ;
 // display bar
if ($app != $currentApp) 
{
  sfContext::getInstance()->switchTo($app);
  $currentRoutes = sfContext::getInstance()->getRouting()->getRoutes() ;
}
sfContext::getInstance()->getLogger()->info('Foo = ' . sfConfig::get('foo') . ' in ' . $app) ;
  // display bar 
sfConfig::set('foo', 'baz') ;
sfContext::getInstance()->getLogger()->info('Foo = ' . sfConfig::get('foo') . ' in ' . $app) ;
  // display baz
/* Reset the app switch if needed  */
if ($app != $currentApp) 
{
  sfContext::getInstance()->switchTo($currentApp);
  $currentRoutes = sfContext::getInstance()->getRouting()->getRoutes() ;
}
sfContext::getInstance()->getLogger()->info('Foo = ' . sfConfig::get('foo') . ' in ' . $currentApp) ;
  // display nothing !!!