is this ISBN-10 Checksum calculation in php correct? - php

Question

I have found a PHP script github ISBN-Calc Routine to perform the ISBN-10 checksum calculation:
<?php
/**
* Calculate ISBN checksum
*
* #param string $isbn
* #return integer
*/
function isbn_checksum($isbn) {
$sum = 0; $isbn = str_split(preg_replace('/[^\d]/', '', $isbn));
foreach($isbn as $key => $z) {
if($key >= 12) break;
$sum += ($key % 2) ? $z * 3 : $z;
}
$checksum = (10 - $sum % 10);
return ($checksum == 10) ? 0 : $checksum;
}
But f.e for my ISBN-10: 0470173424 I get Checksum: 0with this github script.
Accoring to ISBN online checker the checksum should be 4 as is it in the ISBN. Can anyone here provide me with the correct PHP routine, please?
Thanks

Answer 1

That function is for calculating an ISBN-13 check digit, not ISBN-10 - that's why it breaks the loop after the 12th character.
The algorithm for ISBN-10 is different, and requires multiplying the first 9 digits of the number by 10 down to 2. The difference between that sum and the next multiple of 11 is the check-digit. For your example, this would be:
(10 * 0) +
(9 * 4) +
(8 * 7) +
(7 * 0) +
(6 * 1) +
(5 * 7) +
(4 * 3) +
(3 * 4) +
(2 * 2) = 161.
The next multiple of 11 is 165, so the check-digit should be 4 (as you say). In the case where the check-digit would be 10, X is used. We can model this in PHP like this:
function isbn10($isbn) {
$isbn = preg_replace('/[^\d]/', '', $isbn);
$digits = str_split(substr($isbn, 0, 9));
$sum = 0;
foreach ($digits as $index => $digit) {
$sum += (10 - $index) * $digit;
}
$check = 11 - ($sum % 11);
return ($check === 10) ? 'X' : $check;
}
echo isbn10('047017342');
4
You can see this working here: https://eval.in/1039654

Answer 2

The previously marked answer is close but incomplete.
Specifically this portion:
$check = 11 - ($sum % 11); // This can output 1,2,3,4,5,6,7,8,9,10,11 not 0
return ($check === 10) ? 'X' : $check; // This is incomplete does not address 11
The code does not deal with the situation where 11 - 0 = 11. I have tried to clarify it below.
function isbn10($isbn)
{
$isbn = preg_replace('/[^\d]/', '', $isbn);
$digits = str_split(substr($isbn, 0, 9));
$sum = 0;
foreach ($digits as $index => $digit)
{
$sum += (10 - $index) * $digit;
}
$check = 11 - ($sum % 11);
// $check may hold either 10 or 11, but not 0
// 10 becomes X, 11 becomes 0 -- output is 1 character only
if ($check == 10)
{
$check = 'X';
}
elseif ($check == 11)
{
$check = '0';
}
return $check;
}
An example ISBN where the earlier answer fails is 0134093410

Answer 3

There is an library from GitHub: https://github.com/Fale/isbn
There is a function called "Check":
Initialization:
$isbn = new Isbn\Isbn();
Check values: (Example)
$isbn->check->is10('888183718'); // Will return false
$isbn->check->is13('9788889527191'); // Will return true
$isbn->check->is13('978888952719'); // Will return false
You can download the library from the given link.
Maybe that helps a bit.
Have a nice weekend!

Answer 4

If you want to check if the ISBN-10 is correct
Validate ISBN-10
<?php
function isValidIsbn10($isbn) {
$check = 0;
for ($i = 0; $i < 10; $i++) {
if ('x' === strtolower($isbn[$i])) {
$check += 10 * (10 - $i);
} elseif (is_numeric($isbn[$i])) {
$check += (int)$isbn[$i] * (10 - $i);
} else {
return false;
}
}
return (0 === ($check % 11)) ? 1 : false;
}
var_dump( isValidIsbn10('0470173424') );
Source: https://stackoverflow.com/a/14096142/5201919
Will show
1 for true
Demo
https://eval.in/1053913