/**
* Polls. A cache entry is considered valid if the following conditions are met:
*
* - The cache file exists.
* - Its modification time is not less than {@link $mtime}
* - It has not expired yet according to {@link $ttl}
*
* @return bool
*/
protected function isValid()
{
return file_exists($this->path) and filemtime($this->path) >= $this->mtime and filemtime($this->path) + $this->ttl > I::time();
}