code: protected function rotateFiles()
      {
          $file = $this->logFile;
          for ($i = $this->maxLogFiles; $i >= 0; --$i) {
              // $i == 0 is the original log file
              $rotateFile = $file . ($i === 0 ? '' : '.' . $i);
              if (is_file($rotateFile)) {
                  // suppress errors because it's possible multiple processes enter into this section
                  if ($i === $this->maxLogFiles) {
                      @unlink($rotateFile);
                      continue;
                  }
                  $newFile = $this->logFile . '.' . ($i + 1);
                  $this->rotateByCopy ? $this->rotateByCopy($rotateFile, $newFile) : $this->rotateByRename($rotateFile, $newFile);
                  if ($i === 0) {
                      $this->clearLogFile($rotateFile);
                  }
              }
          }
      }
docs: Rotates log files.