如何用PHP重写Java的MD5签名字符串生成代码？

<?php
class SignatureGenerator {
    public function getEncryptStr(string $uuid, string $app_key, string $app_secret, string $time_millis, int $move_card): string
    {
        $encrypt_str = $uuid . $app_key . $app_secret . $time_millis;
        $encrypt_byte = utf8_decode($encrypt_str); // 将UTF-8解码为ISO-8859-1

        $change_str = $this->change($encrypt_byte, $move_card);
        $merge_str = $this->mergeByte($encrypt_byte, $change_str);

        return md5($merge_str);
    }

    private function change(string $encrypt_str, int $move_card): string
    {
        $len = strlen($encrypt_str);
        for ($i = 0; $i < floor($len / 2); $i++) {
            $temp = $encrypt_str[$i];
            $encrypt_str[$i] = $encrypt_str[$len - 1 - $i];
            $encrypt_str[$len - 1 - $i] = $temp;
        }
        return $encrypt_str;
    }

    private function mergeByte(string $encrypt_byte, string $change_str): string
    {
        return $encrypt_byte . $change_str;
    }
}

$generator = new SignatureGenerator();
$uuid = 'your_uuid';
$app_key = 'your_app_key';
$app_secret = 'your_app_secret';
$time_millis = 'your_timestamp';
$move_card = 10; // 移动基数

$signature = $generator->getEncryptStr($uuid, $app_key, $app_secret, $time_millis, $move_card);
echo "Generated Signature: " . $signature;
?>