问为什么神奇的方法'__get‘被调用了两次？

EN

<?php

class Foo {
    private $bar;

    function __get($name){
        echo "__get(".$name.") is called!\n";
        debug_print_backtrace();
        $x = $this->$name;
        return $x;
    }

    function __unset($name){
        unset($this->$name);

        echo "Value of ". $name ." After unsetting is \n";
        echo $this->$name;
        echo  "\n";
    }
}
echo "Before\n";
$foo = new Foo;
echo "After1\n";
unset($foo->bar);
echo "After2\n";
echo $foo->bar;
echo "After3\n";
echo $foo->not_found;
?>

Before
After1
Value of bar After unsetting is
__get(bar) is called!
#0  Foo->__get(bar) called at [E:\temp\t1.php:17]
#1  Foo->__unset(bar) called at [E:\temp\t1.php:24]
PHP Notice:  Undefined property: Foo::$bar in E:\temp\t1.php on line 9

After2
__get(bar) is called!
#0  Foo->__get(bar) called at [E:\temp\t1.php:26]
__get(bar) is called!
#0  Foo->__get(bar) called at [E:\temp\t1.php:9]
#1  Foo->__get(bar) called at [E:\temp\t1.php:26]
PHP Notice:  Undefined property: Foo::$bar in E:\temp\t1.php on line 9
After3
__get(not_found) is called!
#0  Foo->__get(not_found) called at [E:\temp\t1.php:28]
PHP Notice:  Undefined property: Foo::$not_found in E:\temp\t1.php on line 9

 return $this->$name;

 echo $foo->bar;

    class Foo {
        private $bar;

        function __get($name){
            echo "__get is called!";
            return $this->$name;  *** here ***
        }

        function __unset($name){
            unset($this->$name);
        }
    }
    $foo = new Foo;
    unset($foo->bar);
    echo $foo->bar;   *** and here ***