code: private function readNumber()
      {
          // Open
          if (false === $this->handle = fopen($this->filePath, 'c+')) {
              throw new RuntimeException("Failed to open file {$this->filePath}.");
          }
          // Exclusive lock
          if (!flock($this->handle, LOCK_EX)) {
              throw new RuntimeException("Failed to lock file {$this->filePath}.");
          }
  
          return fread($this->handle, $this->length);
      }
docs: Reads the previous number.
  
  @return bool|string