original_string: public static function isValidISBN13($isbn)
      {
          $isbn = self::normalizeISBN($isbn);
          if (strlen($isbn) != 13) {
              return false;
          }
          return
              substr($isbn, 12) == self::getISBN13CheckDigit(substr($isbn, 0, 12));
      }
code: public static function isValidISBN13($isbn)
      {
          $isbn = self::normalizeISBN($isbn);
          if (strlen($isbn) != 13) {
              return false;
          }
          return
              substr($isbn, 12) == self::getISBN13CheckDigit(substr($isbn, 0, 12));
      }
code_tokens: public static function isValidISBN13 ( $ isbn ) { $ isbn = self :: normalizeISBN ( $ isbn ) ; if ( strlen ( $ isbn ) != 13 ) { return false ; } return substr ( $ isbn , 12 ) == self :: getISBN13CheckDigit ( substr ( $ isbn , 0 , 12 ) ) ; }
docstring: Is the provided ISBN-13 valid?
  
  @param string $isbn The ISBN-13 to test.
  
  @return boolean
docstring_tokens: Is the provided ISBN - 13 valid?