<?php
/** 空值不参与，字典序，MD5 小写 */
function build_sign(array $params, string $secretKey): string {
    unset($params['Sign']);
    ksort($params, SORT_STRING);
    $parts = [];
    foreach ($params as $k => $v) {
        if ($v === null) {
            continue;
        }
        $s = (string)$v;
        if ($s === '') {
            continue;
        }
        $parts[] = $k . '=' . $s;
    }
    $raw = implode('&', $parts) . '&SecretKey=' . $secretKey;
    return md5($raw);
}