code: public function minLength($n)
      {
          $this->constraints[] = (function (Password $str) use ($n) {
              if (mb_strlen($str) < $n) {
                  return sprintf("Minimum password length is %d characters.", $n);
              } else {
                  return true;
              }
          });
          return $this;
      }
docs: The minimum length in characters
  
  Unicode code point is counted as a single character (NIST recommendation)
  
  @param int $n
  @return $this