In line 260, isset($this->gtin14), isset($this->isbn13) & isset($this->isbn10) always evaluates to TRUE, so the function will only work when given a gtin14 number. If isbn13 or isbn10 is given then function will fail. Maybe all isset() calls has to be removed from line 260
Keith Nunn - 2009-11-19 20:57:20 - In reply to message 1 from Kashif Hisam
It looks like the error is actually in the gtin-14 check. I've fixed that and the valid_isbn() function seems to work fine now. I'll upload the fix in a minute. Sorry for the long delay.