crockford32_normalize.php

<?php

function crockford32_normalize(string $string): string|null {
    $string = strtoupper($string);
    $string = str_replace(
        search: ["O", "I", "L", "U"], // this U replacement is non-standard
        replace: ["0", "1", "1", "V"],
        subject: $string,
    );
    if (preg_match("/[^0-9A-HJKMNP-TV-Z]/", $string)) {
        return null; // input string contained invalid chars
    }
    return $string;
}