code: private function getChangedAt(string $filename) : float
      {
          clearstatcache(true, $filename);
  
          return file_exists($filename)
              ? (float) file_get_contents($filename)
              : -1;
      }
docs: Get the content of a file as a timestamp.
  
  Reading the timestamp from a file is much more reliable than filemtime.
  Especially on virtual machine shared folders due to the lag in updating the file stats on the client machine.
  
  @param string $filename
  
  @return float