code: protected function generateNumber($number)
      {
          if (!empty($this->prefix)) {
              if (0 !== strpos($number, $this->prefix)) {
                  $number = 0;
              } else {
                  $number = intval(substr($number, strlen($this->prefix)));
              }
          } else {
              $number = intval($number);
          }
  
          if ($this->debug && 999999 > $number) {
              $number = 999999;
          }
  
          return $this->prefix . str_pad($number + 1, $this->length - strlen($this->prefix), '0', STR_PAD_LEFT);
      }
docs: Generates the number.
  
  @param string $number
  
  @return string