frontendConfiguration.class.php

<?php

class frontendConfiguration extends sfApplicationConfiguration
{
  public function configure()
  {
    $this->dispatcher->connect('template.filter_parameters', array($this, 'filterTemplateParameters'));
  }

  public function filterTemplateParameters(sfEvent $event, $parameters)
  {
    $parameters['variable_name'] = 'variable_value';

    return $parameters;
  }
}