Encode json with cURL to an API help

Hello Guys,
I am having a little problem with a Unifi Controller that I am trying to communicate with through PHP.
The API documentation for the controller is located here: http://dl.ubnt.com/unifi/4.6.0/unifi_sh_api

And the function I'm trying to use is to authenticate a user.
# cmd/stamgr
# authorize-guest(mac, minutes, [up=kbps, down=kbps, bytes=MB])
unifi_authorize_guest() {
    if [ $# -lt 2 ] ; then
        echo "Usage: $0 <mac> <minutes> [up=kbps] [down=kbps] [bytes=MB] [ap_mac=mac]"
        return
    fi

    mac=$1
    minutes=$2
    other_payload=`named_args_to_payload "$@"`

    ${curl_cmd} --data "json={'cmd':'authorize-guest', 'mac':'${mac}', 'minutes':${minutes}${other_payload}}" $baseurl/api/s/$site/cmd/stamgr
}

I got it working with the first 3 commands, but I cant get upload, download and quota to work.
This is how I do it now:

$data = json_encode(array(
                                    'cmd'=>'authorize-guest',
                                    'mac'=>$id,
                                    'minutes'=>$minutes));

I hoped that I could just do it like this, but it doesnt work :-( Am I dont know about this kind of programmering.

$data = json_encode(array(
                              'cmd'=>'authorize-guest',
                              'mac'=>$id,
                              'minutes'=>$minutes,
                              'up'=>$upload,
                              'down'=>$download));

So I hope some of you guys can help me out here, and tell me how the string to the controller should look like to get it working.

Thanks,