public function clearCache($ttl = self::DEFAULT_CACHE_TTL)

  {

  $ttl = (int) $ttl;

  $dir = escapeshellcmd($this->_cacheDir);

  $command = "find {$dir} \! -type d -amin +{$ttl} -exec rm -v '{}' ';'";

  exec($command, $stringOutput);

  return $stringOutput;

  }