Convert GLS track ID to GLS reference

convert-gls-trackid.php

<?php

// based on https://stackoverflow.com/questions/10471991/convertions-between-decimal-and-base-36#10472259

function convertReferenceToTrackID($number) {
    $offset = 2721109907456;
    $base = 36;
    $in = (string) ($number + $offset);
    $out = '';

    for ($i = strlen($in) - 1; $i >= 0; $i--) {
        $out = base_convert(bcmod($in, $base), 10, $base) . $out;
        $in = bcdiv($in, $base);
    }

    return strtoupper(preg_replace('/^0+/', '', $out));
}

function convertTrackIDToReference($trackID) {
    $offset = 2721109907456;
    $base = 36;
    $out = '';

    for ($i = 0, $l = strlen($trackID); $i < $l; $i++) {
        $x = base_convert(substr($trackID, $i, 1), $base, 10);
        $out = bcadd(bcmul($out, $base), $x);
    }
    $out = (int) $out;
    return $out - $offset;
}

var_dump(convertReferenceToTrackID(71642028539));
var_dump(convertTrackIDToReference('ZMZ0E96J'));