php classのオブジェクトをInstantiateしてClass内Public VariableのEchoの仕方

<?php
    $name = "Nobu Kim";
    $age = 42;

    $user1 = new User('Nobu', '42');
    echo $user1->username; //doesn't get printed
    echo $user1->age; //doesn't get printed
    //print_r($user1);
    //test->__destruct(); cannot call explicitly
    echo "wtf"; //gets printed
    echo $name; //gets printed
    //echo $user1;

    class User
    {
        public $username;
        public $age;

        function __construct($name, $age)
        {
            //Constructor statements here
            $username = $name;
            echo $username; //gets printed
            $age = $age;
            echo $age; //gets printed
        }
    }


        function __destruct()
        {
            //Destructor code here
        }



?>  

上記、何故コメントにて”Doesn't get printed"のライン、echo $user1->username & echo $user1->age;はプリントされないのでしょうか。