Be Excellent To Each Other

IF $forum_id is not set THEN
    set $forum_id = 0

Set $hash = the first 10 characters of the MD5 hash of $user_id and $forum_id joined by a hyphen (-) (ie. if $user_id was 16 and $forum_id was 5, take the MD5 hash of "16-5"), using the key 'stupidASP'
Set $link = "http://www.beex.co.uk/" and $user_id / $forum_id / $hash

if (!$forum_id ) {
    $forum_id = 0;   
}
$hash = hash_hmac("md5", $user_id."-".$forum_id, 'stupidASP');
$hash = substr($hash, 0, 10);
$link = 'http://www.beex.co.uk/'.$user_id.'/'.$forum_id .'/'.$hash;

if (!forum_id ) {
    forum_id = 0;   
}

public string CalculateMD5Hash(string input)
{
    // step 1, calculate MD5 hash from input
    MD5 md5 = System.Security.Cryptography.MD5.Create();
    byte[] inputBytes = System.Text.Encoding.ASCII.GetBytes(input);
    byte[] hash = md5.ComputeHash(inputBytes);

    // step 2, convert byte array to hex string
    StringBuilder sb = new StringBuilder();
    for (int i = 0; i < hash.Length; i++)
    {
        sb.Append(hash[i].ToString("X2"));
    }
    return sb.ToString();
}
An example call:

string hash = CalculateMD5Hash("abcdefghijklmnopqrstuvwxyz");
…returns a string like this:

C3FCD3D76192E4007DFB496CCA67E13B

    public string hash_hmac(string message, string key)
    {

        System.Text.ASCIIEncoding encoding = new System.Text.ASCIIEncoding();
        byte[] keyByte = encoding.GetBytes(key);


        System.Security.Cryptography.HMACMD5 hmacmd5 = new System.Security.Cryptography.HMACMD5(keyByte);

        byte[] messageBytes = encoding.GetBytes(message);

        byte[] hashmessage = hmacmd5.ComputeHash(messageBytes);

        string hashmessageString = "";

        for (int i = 0; i < hashmessage.Length; i++)
        {
            hashmessageString += hashmessage[i].ToString("X2"); // hex format
        }
        return (hashmessageString);
    }

        int forum_id = 5;
        int user_id = 16;

        if (forum_id == null)
            forum_id = 0;

        string hash = hash_hmac(String.Format("{0)-{1}", user_id, forum_id), "stupidASP").Substring(0, 10);
        string link = String.Format("http://www.beex.co.uk/{0)/{1}/{2}", user_id, forum_id, hash);