code: public function unlock()
      {
          try {
              // remove the PID from the PID file if set
              if ($this->pid === $this->getSerial() && is_resource($this->fh)) {
                  // remove the PID from the file
                  $this->removeLineFromFile($this->pid, $this->fh);
  
                  // finally unlock/close the PID file
                  flock($this->fh, LOCK_UN);
                  fclose($this->fh);
  
                  // if the PID file is empty, delete the file
                  if (filesize($filename = $this->getPidFilename()) === 0) {
                      unlink($filename);
                  }
              }
          } catch (FileNotFoundException $fnfe) {
              $this->getSystemLogger()->notice(sprintf('PID file %s doesn\'t exist', $this->getPidFilename()));
          } catch (LineNotFoundException $lnfe) {
              $this->getSystemLogger()->notice(sprintf('PID %s is can not be found in PID file %s', $this->pid, $this->getPidFilename()));
          } catch (\Exception $e) {
              throw new \Exception(sprintf('Can\'t remove PID %s from PID file %s', $this->pid, $this->getPidFilename()), null, $e);
          }
      }
docs: Remove's the UUID of the actual import process from the PID file.
  
  @return void
  @throws \Exception Is thrown, if the PID can not be removed