src/Isbn.php
public function to13() : Isbn
    {
        if ($this->is13) {
            return $this;
        }

        return new Isbn(Internal\Converter::convertIsbn10to13($this->isbn), true);
    }
public function to13() : Isbn
    {
        if ($this->is13) {
            return $this;
        }

        return new Isbn(Internal\Converter::convertIsbn10to13($this->isbn), true);
    }
public function to13 ( ) : Isbn { if ( $ this -> is13 ) { return $ this ; } return new Isbn ( Internal \ Converter :: convertIsbn10to13 ( $ this -> isbn ) , true ) ; }
Returns a copy of this Isbn, converted to ISBN-13.

@return Isbn The ISBN-13.
Returns a copy of this Isbn converted to ISBN - 13 .