This file contains a PHP code to generate a checksum based in Luhn Algorithm.

luhn_checksum_generator.php

/**
 * This file contains a PHP code to generate a checksum based in Luhn Algorithm.
 * To learn more about Luhn algorithm: https://en.wikipedia.org/wiki/Luhn_algorithm
 * To validate the generated code: https://www.dcode.fr/luhn-algorithm
 *
 * @param  string  $code
 * @return int
 * 
 * Example: $code = '7992739871'
 *          function returning: 3
 */
public function luhnChecksum($code)
{
    $digits = array_reverse(str_split($code));

    foreach ($digits as $index => $value) {
        if($index % 2 === 0) {
            $digits[$index] *= 2;
            
            if($digits[$index] > 9) {
                $digits[$index] -= 9;
            }
        }
    }

    $mod = array_sum($digits) % 10;

    return $mod > 0 ? 10 - $mod : 0;
}