Piwik\Plugins\CustomPiwikJs\File::hasWriteAccess PHP Method

    public function hasWriteAccess()
    {
        if (file_exists($this->file) && !is_writable($this->file)) {
            return false;
        }
        return is_writable(dirname($this->file)) || is_writable($this->file);
    }

 public function update()
 {
     if (!$this->toFile->hasWriteAccess() || !$this->fromFile->hasReadAccess()) {
         return;
     }
     $newContent = $this->getUpdatedTrackerFileContent();
     if ($newContent !== $this->getCurrentTrackerFileContent()) {
         $this->toFile->save($newContent);
     }
 }