从User类对象获取ObjectId

I am getting object of Pointer<_User> column field in shape of associative array. Now I want to get Object Id of that User from array,

But when I tried to do this I get an error.

The array looks like this:

Parse\ParseUser Object
(
    [_sessionToken:protected] => 
    [serverData:protected] => Array
        (
            [__type] => Object
            [addallFBfriends] => NO
            [autoacceptfriendrequest] => YES
            [betsCreated] => 16
            [birthday] => DateTime Object
                (
                    [date] => 2015-04-09 07:31:18
                    [timezone_type] => 2
                    [timezone] => Z
                )

            [className] => _User
            [credit] => 77
            [earning] => 4
            [email] => sim1@sim1.com
            [firstname] => sim
            [friends] => Parse\ParseRelation Object
                (
                    [parent:Parse\ParseRelation:private] => Parse\ParseUser Object
 *RECURSION*
                    [key:Parse\ParseRelation:private] => friends
                    [targetClassName:Parse\ParseRelation:private] => _User
                )

Now suppose I want to get firstname from that array so please help me out that how could I do this.

try this...

 $currentUser  = ParseUser::getCurrentUser();
    if ($currentUser ) {
        // do stuff with the user
            echo "|||username=  ".$currentUser ->username;
            echo "|||email=  ".$currentUser ->email;
            echo "|||ObjectId=  ".$currentUser ->getObjectId();
            $sessionToken = ParseUser::getCurrentUser()->getSessionToken();
            echo "|||sessionToken=  ".$sessionToken;
        //print_r($currentUser);
    } else {
        // show the signup or login page
            echo "||no current user please login";
     }