php 5.6.13 - testing empty and __get() behavior

testing empty() and __get() behavior

<?php

class Xpto
{
    protected $a = 'a';
    public $b = 'b';
    public function __get($key)
    {
        echo '__get() ';

        return $key;
        //var_dump($key);
    }
}
$obj = new Xpto();
var_dump(empty($obj->a)); #bool(true) 
var_dump($obj->a); # __get() string(1) "a" 
var_dump(empty($obj->b)); #bool(false) 

class Xpto
{
    protected $a = 'a';
    public $b = 'b';
    public function __get($key)
    {
        echo '__get() ';

        return $key;
        //var_dump($key);
    }

    public function __isset($key)
    {
        return $key;
    }
}
$obj = new Xpto();
var_dump(empty($obj->a)); #__get() bool(false)