public static function isValidISBN13($isbn)
    {
        $isbn = self::normalizeISBN($isbn);
        if (strlen($isbn) != 13) {
            return false;
        }
        return
            substr($isbn, 12) == self::getISBN13CheckDigit(substr($isbn, 0, 12));
    }
public static function isValidISBN13($isbn)
    {
        $isbn = self::normalizeISBN($isbn);
        if (strlen($isbn) != 13) {
            return false;
        }
        return
            substr($isbn, 12) == self::getISBN13CheckDigit(substr($isbn, 0, 12));
    }
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 ) ) ; }
Is the provided ISBN-13 valid?

@param string $isbn The ISBN-13 to test.

@return boolean
Is the provided ISBN - 13 valid?