What is the purpose of converting a UPC to an ISBN13 in PHP?

Converting a UPC to an ISBN13 in PHP can be useful when working with different types of product identifiers. The UPC (Universal Product Code) is a 12-digit barcode used primarily in the United States, while the ISBN13 (International Standard Book Number) is a 13-digit code used for identifying books. By converting a UPC to an ISBN13, you can standardize the product identifier format for books and make it easier to work with book-related data.

<?php
function convertUPCtoISBN13($upc) {
    // Add the prefix '978' to the UPC to create a 13-digit ISBN13
    $isbn13 = '978' . substr($upc, 0, 9);

    // Calculate the check digit for the ISBN13
    $sum = 0;
    for ($i = 0; $i < 12; $i++) {
        $sum += $i % 2 ? 3 * $upc[$i] : $upc[$i];
    }
    $check_digit = (10 - ($sum % 10)) % 10;

    return $isbn13 . $check_digit;
}

$upc = '123456789012';
$isbn13 = convertUPCtoISBN13($upc);
echo $isbn13;
?>