求解释一下这段php代码

代码如下

 $secret = "XXXXXXXXXXXXXXX"; 
 $username = $_POST["username"];
$password = $_POST["password"]; 
if (!empty($_COOKIE["getmein"])) {
   if (urldecode($username) === "admin" && urldecode($password) != "admin") {
        if ($COOKIE["getmein"] === md5($secret . urldecode($username . $password))) {
            echo "Congratulations! You are a registered user.\n";
            die ("The flag is ". $flag);
        }
         else {
            die ("Your cookies don't match up! STOP HACKING THIS SITE.");
         }
    }
    else {
        die ("You are not an admin! LEAVE.");
        }
    }

主要是 if ($COOKIE["getmein"] === md5($secret . urldecode($username . $password))) 这句怎么理解？$secret.urldecode($username . $password)，这里的函数调用方式怎么这么奇怪？那个 " . "是什么意思？

你好，意思是，COOKIE信息是md5是加密后的字符串，$secret.urldecode(...)中那个点是字符串拼接

那个点是字符串连接符，拼接字符串用的

不是在调用函数，而是让$secret和后面的值拼接