I found another way to do it. in your unity3d script, make a function that call an external javascript that send data to unity3d.

so in unity3d.. on start() method

Application.ExternalCall("setUserID", 0);

on browser

function setUserID(Void)
{   
     GetUnity().SendMessage( "webConnect", "setSessionId", "<?php echo $session['session_id']; ?>" ); 
}

and then in unity3d, on the same script as the first call, create another function

void setSessionId(string sessionString)
{
    try
    {
        _user.setSessionId(sessionString);
        guitext.setSessionText();
    }
    catch { Debug.Log("null session"); } 
}

if you do this, the javascript call will only be called after the unity instance fully loaded

this is because the web player have not fully loaded, and yet you call the sendmessage function. there is some tutorial somewhere, i also am looking for it while i am writing this post.

i will post the link if i find it.