jQuery + php How do I return multiple values in my data object?

I have my simple jquery & php reg/login script working. Basically if the user's name and pass is in the DB the php echoes out true and in my javascript I have

I want to pass the username back to my javascript and save it in a variable, how can I do that?

I think I can something like this in my php:

json_encode(array("boolean" => "true", "username" => '$username'));

but how do I handle it back in my jQuery?

$('#login').click(function(e){
      $.get("login.php",{username: $("#username").val(), password: $("#password").val()},function(data){
         if(data == "true"){

            alert("Logged in");
            $('#logindiv').slideUp();
            $('#gamediv').show(); 
            $('#registerdiv').hide();
            $('#gameheader').html(userloggedin);
         }else{
            alert("Not Logged in");
         }
      });
});

EDIT: login.php

if (isset($_GET['username'])&& isset($_GET['password'])) {
$username = $_GET['username'];
    $password = $_GET['password'];

$sql = mysql_query("SELECT * FROM members WHERE username='$username' AND password='$password'") or die(mysql_error()); 

       // Get member ID into a session variable
        $id = $row["id"];   
        session_register('id'); 
        $_SESSION['id'] = $id;
        // Get member username into a session variable
        $username = $row["username"];   
        session_register('username'); 
        $_SESSION['username'] = $username;
        $json = array("boolean" => "true", "username" => $username);

        echo(json_encode($json));
        //exit();

} else {
    echo "false";
}

main.html ...

$('#login').click(function(e){
      $.getJSON("login.php",{username: $("#username").val(), password: $("#password").val()},function(data){
         if(data.username!==undefined){
            alert(data.username);
            $('#logindiv').slideUp();
            $('#gamediv').show(); 
            $('#registerdiv').hide();
            $('#gameheader').html(data['username']);
         }else{

         }
      });
});