[PHP] GTIN validator

gtin_validator.php

<?php

function gtinIsValid($gtin) {
    $gtin = str_pad(trim($gtin), 14, "0", STR_PAD_LEFT);
    $checkSum = 0;
    $gtinBody = substr($gtin, 0, strlen($gtin) - 1);
    $lastDigit = substr($gtin, -1);
    
    foreach(str_split($gtinBody) as $i => $char) {
        if ($i % 2 == 1) {
            $checkSum += intval($char);
        } else {
            $checkSum += (3 * intval($char));
        }
    }
    
    $checkDigit = (10 - ($checkSum % 10)) % 10;
    
    return $checkDigit === intval($lastDigit);
}