How to do MD5 with foreign characters?

Question

So I am making my custom highscore board in mobile game, using unity engine.

I setup my mysql database, and bought highscore asset from store, it works, but only with english user name. So basically it send user's name, score to .php script.

But I want that script also can receive korean characters as user's nickname. My users will use korean characters too as nickname not only english characters.

How can I achieve this?

Here are codes.

------------------(Highscore.cs at unity side)

WWWForm rsFm = new WWWForm();
            rsFm.AddField("name",name);   
        // at here name field, I want to receive korean characters as well.
            rsFm.AddField("tables",tables);
            rsFm.AddField("hash",GetHash(name));
            WWW rs = new WWW(registerUserURL,rsFm);
            yield return rs;

..................

string GetHash(string usedString){ //Create a Hash to send to server
    MD5 md5 = MD5.Create();
    byte[] bytes = Encoding.ASCII.GetBytes(usedString+secretKey);
    byte[] hash = md5.ComputeHash(bytes);

    StringBuilder sb = new StringBuilder();
    for(int i = 0; i < hash.Length; i++){
        sb.Append(hash[i].ToString("x2"));
    }
    return sb.ToString();

}

RegisterUser.php

<?php
include('ServerConnect.php');
$connection = Connect();//Attempt to Connect to MYSQL Server & DataBase

//Get variables from unity
$name = $_POST['name'];
$date = strtotime(date("Y-m-d"));
$tables = $_POST['tables'];
//Security Check
$hash = $_POST['hash'];
if(CheckKey($hash,$name) == 'fail'){ //Check if hash is valid
    echo 'Security Failure'; exit;

}

ServerConnect.php

function CheckKey($hash,$name){  //Check weather Md5 hash matches
    global $secretKey; 
    $checkHash = md5($name.$secretKey);
    if(strcmp($checkHash,$hash) == 0){
        return 'pass'; //hash matches
    }else{
        return 'fail'; //hash failed
    }

}

When I input korean character and send, console Result says "Security Failure" at above code.