EAN13 barcodes are commonly used to label products in Europe. If you want to know more about how they work then please view the Wikipedia entry on European Article Numbers.

EAN13 barcodes are actually 12 digits long and are validated by using a check digit, which is placed at the end, making the code 13 digits long. The check digit is worked out by the following process:

- Add up all of the even numbers and multiply this number by 3.
- Add up all of the odd numbers and add this result to the result of the even numbers.
- Divide the number by 10 and keep the remainder (modulo).
- If the remainder is not 0 then subtract 10 from this number.

Here is a function that runs through those steps, but also check to see what length the barcode is. If it is 13 digits long then it returns both the original check digit and the calculated check digit. If the barcode is 12 digits long then it returns the checksum. In both cases the original barcode is also returned.

```
function validateEan13($digits)
{
$originalcheck = false;
if ( strlen($digits) == 13 ) {
$originalcheck = substr($digits, -1);
$digits = substr($digits, 0, -1);
} elseif ( strlen($digits) != 12 ) {
// Invalid EAN13 barcode
return false;
}
// Add even numbers together
$even = $digits[1] + $digits[3] + $digits[5] + $digits[7] + $digits[9] + $digits[11];
// Multiply this result by 3
$even = $even * 3;
// Add odd numbers together
$odd = $digits[0] + $digits[2] + $digits[4] + $digits[6] + $digits[8] + $digits[10];
// Add two totals together
$total = $even + $odd;
// Calculate the checksum
// Divide total by 10 and store the remainder
$checksum = $total % 10;
// If result is not 0 then take away 10
if($checksum != 0){
$checksum = 10 - $checksum;
}
// Return results.
if ( $originalcheck !== false ) {
return array('barcode'=>$digits, 'checksum'=>$checksum, 'originalcheck'=>$originalcheck);
} else {
return array('barcode'=>$digits, 'checksum'=>$checksum);
}
}
```

To test this I ran a few codes through the function.

```
// two normal barcodes
print_r(validateEan13(5023920187205));
print_r(validateEan13(5010548001860));
// one short barcode to work out checksum
print_r(validateEan13(501054800186));
// a normal barcode
print_r(validateEan13(5034504935778));
// the same barcode with a broken number
print_r(validateEan13(5034504735778));
// two random numbers, one of which is not long enough to be an EA13 barcode
print_r(validateEan13(7233897438712));
var_dump(validateEan13(3345345345));
```

This prints out the following results, which I have annotated for your convenience.

```
// two normal barcodes
Array
(
[barcode] => 502392018720
[checksum] => 5
[originalcheck] => 5
)
Array
(
[barcode] => 501054800186
[checksum] => 0
[originalcheck] => 0
)
// one short barcode to work out checksum
Array
(
[barcode] => 501054800186
[checksum] => 0
)
// a normal barcode
Array
(
[barcode] => 503450493577
[checksum] => 8
[originalcheck] => 8
)
// the same barcode with a broken number
Array
(
[barcode] => 503450473577
[checksum] => 4
[originalcheck] => 8
)
// two random numbers, one of which is not long enough to be an EA13 barcode
Array
(
[barcode] => 723389743871
[checksum] => 4
[originalcheck] => 2
)
bool(false)
```